I've got a Glock 23 also and love it. Very accurate, durable and after several thousand rounds through it...no problems whatsoever. I've also shot my friend's USP .40 and loved it. Same as the glock...very accurate and I don't think he's had a single problem w/ it. B/w the two I went for the glock b/c it was 300 bucks cheaper and yet performed almost identical to the USP, so I didn't feel the $ was worth it. The Sig is also a nice shooting gun, but I don't like it as much as the previous two b/c it feels too "thick" in my hand. Not necessarily shooting it was uncomfortable, but there was no way I would ever carry that thing. It's just too bulky for a conceal carry (which is the real reason I bought a pistol in the first place). The glock is much slimmer and fits very well w/ my inside-the-pants holster. I shot the S&W a few years ago and don't remember anything bad about it, but I don't remember anything great about it. S&W is a great company, just nothing really sticks in my mind about that pistol, good or bad. I haven't shot an XD, but I haven't heard anything bad about them. Everyone who owns one seems to love it.
Personally, if you have the opportunity, I would try to shoot every pistol in your "wish list" and then decide. USPs are super sweet and they're in Counter Strike so they've gotta be cool...then you shoot one for real and you hate it (I seriously doubt that happening, but just an example). Point being, don't buy a pistol based on name recognition solely (if you can help it). Some people massively hate Glocks and think they're POS, ok fine that's they're opinion. We can give you OUR opinions on pistols, but the only real way you can know if you'll like something is if you shoot it. The feel is important...a Glock could be really uncomfortable in your hand while the USP fits perfectly or vice versa...you won't know until you at least hold them. If you don't have the opportunity to shoot each one, then at least go hold each one and see how it feels...that is important. If it's uncomfortable to shoot, you eventually will not shoot it enough/as much as you like. That's just natural. Shooting should be enjoyable, not an ass pain.