I was recently given an HK USP in 9mm. I don't even have any 9mm ammunition at the moment (I own a WASR-10, Mauser K98 and a .22 revolver whos maker I literally do not know) so the only thing I've been able to do is look at it and play around with the mechanisms.
First thing I noticed was when I pulled the slide back it had the sturdiest, most mechanical (read: robotic) feel I've ever gotten out of a handgun. Compared to my AK-knokoff it was an entirely different dimension. And the same experience was repeated with the decocking lever, the magazine ejection/loading. Super-precise machining, it would seem.
The other handguns I have any clear memory of using are:
Glock 17, Glock 20 and several Glocks I don't quite remember. HK has the same weight ratio weirdness as the Glocks, but you get used to that. The gun itself is much more ergonomic and slim, and it fits my hand better than any gun I've held with one exception.
Walther P99: It was actually a S&W P99, but they're the same thing and I like Walther better so there. But anyways, the P99 handled exactly like this USP does except the Walther had nicer grips and didn't seem as click-clak sharp as the USP. That may just be because the P99 was more worn in, though.
The requisite Colt 1911s, from various sources. Mostly Colt and Kimber, though. Those things are a completely different feel. They're heavy as all Hell, and the trigger pull is probably the best I've gotten on anything outside of my Mauser K98 (and comparing a bolt-action sniper rifle to a pistol isn't fair, anyways).
Once I get some rounds and have time to go down to the range or the mountains I'll tell you how it shoots compared to those guns, but so far I'm very impressed with Heckler and Koch.
A note on "Why 9mm?": Aside from the fact that it was a gift, I choose 9mm because I'm not exactly a pistol marksman. I try to aim, and I don't have any flinching problems but I'm currently not well equipped to handle something in .45 or 10mm. I've considered .40 and .357Sig, but none of those have been offered to me yet
Since I'm such a lame shooter I might miss more than a few times. This isn't a huge safety issue, I live by myself and I'm surrounded by a cinder-block wall, so I doubt 9mm rounds are going to be accidentially offing the neighbour's dog. I'm getting a laser sight to help alleviate this and maybe some day when I can actually control a 1911 I'll graduate to a .45 or such, but for the moment it's 9x19.