When I did my four gun comparison thread 4-5 months ago, I included "my take" on all the guns. Here's an updated "my take" on these six guns. Maybe it will spur some debate on the merits on the various designs.
SIG P229R:
Pros: When you pick up this gun you get a real sense of, "Wow... somebody has really thought this thing through." SIG has been working on the 22X design for over 30 years now, and they really seem to have pistol making down to a very refined artform.
One immediate difference between this gun and the others in this thread is the trigger. No metal-on-plastic here. SIG has the best trigger of the bunch by a considerable margin in my view (second best is Beretta). Both the double action and single action are highly usable. You don't have to treat the first DA shot as a near write-off with this gun - the way it is with some guns.
The ergonomics are also very good. The Walther P99 has good ergonomics, but it is so light that you lose some of the benefit of being able to get a good grip on it, because of the felt recoil. The SIG is the second heaviest gun in this comparison (after the Beretta) and is also the most controllable of the group. The HK's recoil reduction system is a neat idea, but good old fashioned weight probably works better.
I also like the sights a lot. The stock front sight on pistols is generally too high for my sight picture, but the SIG came from the factory exactly the way I wanted it. The tops of the sights line-up, and point of aim in on target, when I raise the pistol to my natural shooting position. I'm really liking this gun right now. See my thread on the P229R for more info and some different pics.
Cons: Right now, all I can think of is... the gun is kind of expensive compared to many of it's competitors. The 10th round can also be tricky to get into the magazine, although a GLOCK speed-loader will fit over a SIG mag (and every other mag in this comparison other than an HK).
HK USP:
Pros: If you ask me, the USP is the last word in "reliability", as I understand the meaning of that word. Not that "it will shoot 300,000 rounds without any parts breaking" kind of thing, but simply that this thing functions the way it is supposed to shot after shot after shot, all day, all week, all year. I've only had mine for 6 months or so, but have taken it out every week in that time, and I have never experience a fail to feed, fail to eject, or failure of the slide to lock back - with any ammo - ever.
The gun is also built to be "durable" (different from "reliable" in my mind). I think with the combination of the USP design (including the recoil reduction system) and the 9mm cartridge, this gun will long outlive me.
Cons: It has a somewhat big boxy grip that isn't great for smaller hands. I actually don't mind the grip myself, but I will admit that it is not as good as it could be. I find that I'm sometimes repositioning my strong hand when shooting, and that is not good. The grip isn't terrible, but it could be better.
Not everyone likes the placement of the decocking lever or the mag release. For myself, I ride the decocker with my strong hand thumb. This works well for me. I think the idea of accidentally decocking the gun during firing is largely a myth. With my style of shooting it would be all but impossible.
I think the front sight is too high for the rear sight. Even if you look in the pics you can see that the front post on the USP is higher than the other guns.
My other knock on this gun is the stock trigger. It is pretty decent once you figure out how it works (just keep thinking 'squeeze', 'squeeze', 'squeeze'... not pull), but it's just too much thinking for me to give it full marks. There seems to be some small amount of travel beyond the break point on the single action trigger.

That is just F'd up if you ask me. The trigger should not move past the break point at all without the hammer coming forward. It totally screws up the meaning of "break point". It's hard to explain exactly what I mean, but USP owners will probably know what I am getting at. I have an HK matchtrigger coming to me, and I'm hoping that this will smooth out the trigger and correct the problem, but who knows. This gun is very accurate, but the trigger can rob you of some of that if you are not paying attention and doing it right (movement in the gun before the hammer comes forward). Definitely not as easy to learn to shoot on as say a SIG or a Beretta.
Beretta 92G
Pros: This is my girlfriends gun, so I only have like maybe 300 rounds through it, but this my take. The gun seems pretty tough. My GF's gun is a ex-Vancouver Police gun. It's still going strong. It shoots quite accurately, and it's weight makes felt recoil very low.
Cons: The gun is really rather massive. O.k., felt recoil is low, but this thing is a tank. It's big, it's long, it's heavy. Beretta tries to market that huge cut out in the slide as making failures to eject almost impossible. This is just not the case in our experience (with this gun). You can get stovepipes between the breechface and the barrel. Failures to load are also not unheard of with this gun. Overall realiable is fine, but of the pistols in this thread, it probably rates lowest or second lowest (the other low one would be the Steyr) on reliablity. I also think that the lock-up between the slide and the barrel is a weakness of this design as compared to the "Browning" type designs. There seems to be a fair bit of play there.
Glock 17:
Pros: Absolutely bomb-proof design. Reliable. Simple. Always goes "bang". One of the really interesting things about GLOCKS, is that they don't really seem to wear much... almost not at all. Mine still looks like brand new. And it has been shot alot. These guns also don't really dirty up very much. The Hex barrel is easy to clean. The gun generally is easy to clean and maintain. Easy and safe holster training gun. Instantly recognizable icon. A modern classic.
It should also be mentioned that you get a lot of gun for the $700 to $800 bucks these these are going for now... 2/3 of what many of the HK and SIG guns go for. And if the invest is a big one for you, you can breath easy with buying a GLOCK. You're not likely to have too many surprises.
Cons: Mushy trigger. "Combat accuracy" only. The design is very sound, but the barrel/slide fit is somewhat lose to improve "reliability", this seems to decrease accuracy. Drop a tighter fitting custom barrel like a Bar-Sto in there and accuracy seems to go way up (for me anyway. I haven't tried a Bar-Sto in my G17, but I have tried a couple of G19s with Bar-Sto, and they were noticably more accurate).
Steyr M9-A1:
Pros: Takes the Glock trigger concept and vastly improves upon it. Very crisp trigger with a solid feel to it. Superior ergonomics to the Glock. Cool "Bulldog" appearance. The internals of the gun are also set up so that all the moving pieces are hidden away from the chamber where the gun dirties up. Makes cleaning very easy. It would make repairs more difficult, except that the mechanical part of the frame is designed as a "box" that can be taken out of the grip en masse. A very innovative design. For me this gun also has some kind of weird appeal to it. I just really like it, and like shooting it, even though it can have it's issues on reliability and accuracy (see below).
Cons: The sights are just plain funky. The triangle and the trapazoid only line up on the point of impact when they are directly level with your eye, otherwise you get into this weird guessing game... so you start going just off of the front sight. I was planning to replace mine with 3-dot night sights, but I never went through with it. After about 8 months of shooting this gun I finally started doing quite well with it. It is never going to rival a SIG (or something like a Steyr GB) on accuracy, but it is a very usable design. The weird thing is that, guns seem to trade off accuracy for reliability (for example, a GLOCK is designed to be reliable, a Steyr GB is designed to be accurate... but dirties up quickly and is somewhat unreliable in my view), whereas the Steyr M series guns are not stellar on either accuracy or reliability. I think my M9-A1's problem is that everything is very tight fitting. The chamber is "fully supported" which sounds good on paper, but is totally unnecessary with a 9mm gun, and actually reduces reliability considerably, especially when the gun starts getting dirty. The Steyr M guns do not like being dirty. I don't want to exagerate this. You are very unlikely to have any problems on the first 100 shots after cleaning, but after that I don't know. When you compare this to the USP and the GLOCK which just keep on ticking almost no matter how dirty they get, this is a weakness of the Steyr. This gun also has a very heavy recoil spring that causes the slide not to lock back quite a bit on standard 115gr. ammo. The overall beefiness of this gun (including the fully supported chamber and the heavy spring) make me think that this gun would be better as a .40S&W than as a 9mm. Still a cool gun though.
Walther P99:
Pros: One ofthe most innovative pistols of the last decade in my view: DA/SA striker fired pistol with top mounted decocker; five interchangable front sights; three interchangable backstraps; cocking indicator; loaded chamber indicator, etc. Is there another striker fired DA/SA gun on the market? It has superior accuracy to Glock-type pistols. Very ergonomic, comfortable grip.
Cons: Extremely light polymer frame seems to be the root cause of the very heavy "snap" recoil. The frame is lighter than a GLOCK to me.. and that is light. All polymer guns are top heavy, but this one seems to take it the furthest. One of the harder recoiling modern 9mm pistols in my view. Point of impact seems to rise out beyond 15m, which may also be connected to the "snap" issue. Not everyone seems to like the mag release... and it can be tight, especially when the mag is full. The other weird thing is that, because it is striker fired, unlike every other SA/DA gun the trigger does not move back by itself when the gun goes into SA mode. It just has minimal to non-existent resistance until you reach the SA break-point. You can get used to it, but I prefer the normal hammer fired SA/DA system (which is used on the P22, by the way).
My rankings:
Accuracy:
SIG
HK/Beretta (tie)
Walther
GLOCK
Steyr (in my subjective experience due to the sights - not the gun... I think)
Reliability:
HK
SIG/GLOCK (tie)
Walther
Steyr/Beretta (tie)
Durability:
GLOCK
HK
SIG
Beretta
Steyr/Walther (tie)
Controllability: (Easy to get rapid follow-up shots on target)
SIG
Beretta
Steyr
GLOCK
Walther (some people like the short little SA trigger, but I find the gun moves too much to get good follow-ups)
Sight picture:
SIG (perfect right out of the box... for me at least)
Beretta
Walther (comes with interchangable front sights... still can get it perfect)
HK (front post is too high for me)
GLOCK (GLOCK night sights would be better)
Steyr (could be replaced with 3-dot night sights)
Fun to shoot: (subjective.. but includes felt recoil)
SIG (I could shoot this gun all day)
HK (could move into a tie for first once I get the match trigger installed)
Steyr (I still have a soft spot for this gun)
GLOCK (works well, but feels a little "ordinary" to me. More fun with a holster and barricades)
Beretta (kind of big a heavy)
Walther (hands start feeling the "zing" fairly quickly)
Looks: (purely subjective)
HK (what a gun should look like... to me)
SIG (looks and feels very "professional" to me)
Steyr (this thing is a "bulldog")
GLOCK (its simple lines have a certain elegance to them... but are also sort of "ordinary". G19 looks better)
Walther
Beretta (our Beretta is looking a bit dated... actually they all look a bit dated)