I had a USP 9mm. I traded it for an HK P2000 9mm after about a year. I like the P2000 quite a bit better. My only problem with the P2000 is that the grip is a bit to short for me, and I need the extended mag floorplates (which I have 5 of on order, hopefully just a few more weeks to go).
I think the USP are a very good gun (especially for people of the monster mitt persuasion), but I did have a few issues with it. I thought it was just generally too big and bulky for a 9mm pistol (I appreciate that it was designed around the .40S&W cartridge). The square shaped grip was problematic for me, and the fact the width of the grip (front to back) is more than I like. I felt like I was shifting it around in my hand too much when shooting - and this showed in my results. The pistol is accurate, but if you are shifting your hands around you are not going to have consistency. I had a lot of the "tight
9 shot 
groups, and 1 flyer" syndrome - which is something I have gotten with a few other guns (GLOCK, Steyr), but not the ones I shoot best with (SIG, P7).
The whole issue of whether you ride the decocker with your thumb or squish your thumb up underneath it is another concern for potential buyers... I rode the decocker and that was o.k., but I don't think it provides optimal control over the gun. I like the "new" HK decockers (P2000, P30) on the back a lot better. Then you can just put your strong hand thumb wherever you want and not worry about that decocker/safety lever. The "1911 meets Oberndorfer" thing is kinda weird.
I also felt the sights were way too high off of the top deck. The front sight in particular. There was a whole bunch of space between the top of the slide and the white dot - and to hit point of aim at 10-15 yards I would have to have the front sight at couple millimeters above the rear sight in my sight picture. That is not the case with my SIG 229, P7 PSP or P2000. All those guns have low profile sights. O.k., sure, you can change out the sights on your USP, but that is like 10% of the cost of the whole gun. I don't understand why the USP don't come with the P2000 style sights right out of the factory. Way better.
I also felt that the stock trigger had too much mush/overtravel. I paid $$$ to get a match trigger kit and have a gunsmith install it, but this seemed to not really affect my overall experience with the gun. It still shot well, but with an inconsistency that bothered me. That leads me to think that the stock trigger is decent, and that my problems with the gun were mostly grip and sight picture related. On my first day taking the P2000 out to the range, I shot better with it than I ever did with my USP in a year of trying.
Having said all that, if you are only going to buy one pistol you should seriously look at a SIG 229 or 226. HK has a lot going for it, but I think they are trickier to learn on than a lot of other guns. Depends on what you want it for. USP is extremely reliable (like GLOCK, XD, etc.), but I really think that SIG is every bit as reliable and really pulls away from the duty pistol pack (in a BIG way) in terms of accuracy and shootablility (which I will define as "ease of getting good results").
Anyway, my 2 cents. Don't flame me if you love the USP. I love them too, they look absolutely deadly and If I find a nice deal on a .40 or .45 I might get another one - this is just my honest experience with my 9mm.