Let me just say upfront I love H&K pistols so this review may be a little biased. When you hold one in your hand they just fell like solid, very well built guns. Once you start firing them, your suspicions are confirmed. Here's a quick summary of my experience with the full size H&K USP.
-Reliable. This gun has never had a malfunction
-Polygonal 4.25'' barrel
-Durable "hostile environment" nitrided finish
-The grip is large and has a good 'stippled' finish that help you hold on to the gun when your hands are wet, sweaty, bloody, ect...
-The sights are somewhat recessed into the slide, minimizing the chance of snagging
-It's a very accurate gun with a nice crisp SA trigger
-The trigger guard is very big allowing for firing with or without gloves
-Ambidextrous magazine release (the location is my preferred location, this may be a con for others)
-The slide release may be my favorite of any handgun I own. I shoot with a 'thumbs up' grip and many slide releases get activated by my thumb. This one doesn't and is still large enough to not have to fumble with it when doing mag changes under stress
-Many different options available in terms of what trigger system you want. (my favorite is striker fired, with a DA/SA close behind)
-A proprietary rail is plain stupid. They have resolved this with newer models but any duty gun without a standard 1913 rail is never a good thing in my opinion
-The DA trigger has a somewhat mushy break
-Magazine capacity (15 rounds) is low compared to other guns in its' class.
You're probably sick of reading by now so here's a video of some shooting from the 12m line, a table top review with disassembly/assembly, a discussion of pros and cons, a size comparison with several other pistols, and a chronograph test with popular duty loads:
Hornady 147gr TAP: 985fps, 317 ft/lbs
Winchester 115gr Ranger: 1146fps, 335 ft/lbs energy
Federal 124gr HST: 1152fps, 365 ft/lbs energy
Speer 124gr Gold Dot: 1176fps, 386 ft/lbs energy