The USP is my pistol of choice. Have it in 9, 40, and 45...love them all. For me, the blocky, stippled grip hits home...fits me perfectly, and just feels natural. Accurate, accurate, accurate...can't be more specific about that. Ambi mag release is great, even though almost everyone talks about how crappy it is, and how it's positioned so awkwardly on the pistol. Trigger finger and thumb on both sides of the mag release, and voila...simplest mag drop in the biz. Sometimes, I think that "everyone" says the same thing about the mag release because it's what they hear from "everyone else."
Don't worry about the parts availability thing. The gun never fails. But if you want spare parts, Wolverine can order them for you...supply them with the part numbers that you are looking for (right from the USP owner's manual) and they will get them for you. Numrich gun parts in New York has almost every USP part in stock, and they will ship to us in Canada, although shipping is quite expensive. I have a tonne of spare parts...no biggy to acquire. Just go and ask the SIG guys how they order parts...and good luck finding SIG part numbers. That's right, you guys know what I'm talking about...
Trigger is a bit gritty...I will admit that, but only in DA. SA is perfect. Besides, you can upgrade to a match trigger kit, if you should so desire, and that will pretty much solve your trigger issue. Or even a match/LEM hybrid trigger, like the one found on the USP Combat Competition...best HK trigger, IMO. Again, DA is where you will feel this. If you're not too concerned with DA firing, then you have nothing to worry about. For some competetive shooting sports, the first shot must be fired in DA, so this is where it may affect you...but honestly, it's not as "rough" as it is made out to be. When HK's are brand new, their internals have a gray, anti-seize compund on them. I have found that by cleaning this compund off of the parts and lubricating them with a good gun oil reduces the amount of grittiness felt in the DA pull. Lightening the hammer spring will also help...a factory match hammer spring, or even one from Wolff gunsprings will do the trick.
I love my USP's. I've had P30's as well, and even a couple of P2000's. Sold all the "P" guns, and kept the USP's. For me, they just work. But again, that's just me.
Hope this helps convince you to join the fan club...good luck, brother.