They are very popular for sure, the best deal you can get out there rigth now in the pistol market is the mp9/40 FS kit range or the g17. I love both, and held/shot both.
All polymer frames are in ball park of $500, that means, that a polymer gun made in Germany costs more here due to the cost of importing & availability (offer/demand) and not due to the inherent value of the gun.
Glocks, Mps, PPQ, CZs, all great designs, all have different benefits. Try one, if you don't like it put it back. There is a lot of people buying mps now because #1, its a great deal, #2 are made in the states bla bla bla.
If I could put my hands on a walther ppq I would buy one in a heart beat, which until the end of 2012 was being imported by SW (not anymore, so you might wonder why).
I love the glock 17 and I love the mp9 too, particularly the grip, and the grip angle. I can shoot the g17 very accurately, but it doesnt point naturally (to me), the mp does that, Which one I like best, do not know yet. Most likely will own both shortly... So it all comes down to how it feels rights to YOU, and which one you enjoy shooting more.
Another thing to consider:
There was a lot of internet noise about the mps accuracy, if you look closer you will see that this is the same people talking, literally the same dudes talking in 3 or 4 forums at once, this is food for a google search, then, the magic begins when you put the words mp9 and acurracy in a google search bar, all BS. Lots of people complaining about how barrel twist is making a difference because it is not a 1/10 twist , and they go ON and ON about it (you can see NUTNFANCY and HIKOK45 shooting 1st generations MPS very accurately), so there is that for the accuracy BS and the fact that LOTS of police departments adopted the mps in it's early days with poor trigger reset.
Shortly after the accuracy problems being in online forums you see local gunsmiths joining the conversation claiming they have the solution, A NEW BARREL which is, of course, offered by them. There is no proven data with videos or anything about the accuracy being POOR, and then BEING corrected by a new BARREL. Make your own conclusions on that one.
SW has changed the twist of the barrel to the 1/10 recently. Make your own conclusions on that.
All videos and all reviews online are with the older barrel. Make your own conclusions on that.
You will also find information about the mp tirgger being updated with APEX parts (apex tactical and a google search will bring the site).
MPs triggers have been upgraded along the way and these parts are no longer needed. This is true if you buy a MP9 or 40 that DOES NOT say stainless on the slide.
For a real review, I would go to the good sources and people who know how to shoot well due to experience with other guns and absolute control of pistol shooting fundamentals, and not a newbie who just got his RPAL, for me, that is online on youtube, then I go to the range and compare, then I compare those shooters shooting other guns, then I don't see any problems in gun designs, just bad shooters...
Check out Nutnfancy review about the mp9, then check Hikok45, reviewing the 40c, the 9 PRO. BOTH OF THEM LIKED IT and they both are HARDCORE glock fans, and both of them can shoot anything in any caliber, so there is that.
I would go with the advice from those guys instead from someone who has only 1 gun, most likely it's biased, and knows how to shoot that one only because it's his pet gun.
My 2 cents.
Nutnfancy shooting a first gen, mp9, before the trigger was changed.
http://www.youtube.com/watch?v=OAgRihaXDh4
Hickok45 shooting the 40C (with a broken slide!!!)
http://www.youtube.com/watch?v=OLFAt63aNbY
and the same guy shooting the 9PRO
http://www.youtube.com/watch?v=xp45sjBz-CA
Another reviewer, unknown to me, with a little biased review, but seems knowledgable (comparing g17 and mp9)
http://www.youtube.com/watch?v=roOgtVcDNTo