Well, Glock is surely popular. I can't talk much about it, when I was in the military a single soldier (bachelor) would store his Glock in my arms room so I got familiar with it. Mechanically, nothing special. Someone mentioned they each have their own advantages. Maybe, but some advantages are much more important than others. You may like Glock's customer service (in this day an age that's in short supply) but the H&K USP series has some very significant technological advancements that affect accuracy, maintenance, power, operation. What's to not like. You big boys may not see it, but take a small framed lady and put a Glock in .40 in her hands and then put a H&K USP in .45 in her hands and I guarantee you she'll notice the superb engineering difference of the H&K in practice. My wife, a relatively new but practiced shooter, felt the Glock .40 much more than my full sized USP in .45. For anyone, not just smaller frames or ladies, recovery is significantly important for a self-defense firearm, but I've learned it can affect smaller frames much more than we might expect. Practice helps, but it still takes more.
The hostile environment finish is outstanding. The polygonal barrel seems like just a GW feature, but read up on it and you'll find it comes with a few significant advantages.
I still love it for it's GW effect anyway!
No lands and grooves? Hmm....
To each his own, but on it's technical merits alone the H&K's are hard to beat. It's one reason why the Mark 23 was recently chosen as the SOCOM standard side arm. Just some stats, average 6000 mean rounds between stoppages with both ball and +P ammo, only 2000 is required. In 450 accuracy test firings, average group 1.44 inches, w/ 65 groups < 1", 4 groups of .5", with 5 rounds through the same hole. Far exceeding government requirements.
But, hell, who can live with just ONE gun!
I don't think I'll ever buy a Glock however.
Either way, Happy shooting!