I have a pretty good bit of experience with GURPS, and I can generally agree with the sentiments so far. It's a pretty good system that can model almost any setting, but it can get extremely complicated depending on how many of the rules you want to use. If you just use everything from the Basic Set books it's pretty manageable, but once you start tossing in supplements like Martial Arts it can get pretty complicated, fast. Part of the problem there, for me at least, is there seems to be a nagging feeling that if you ignore a rule, you're not playing the full game. So... you go from getting stabbed, to checking to see if it's a major wound, to seeing if you are knocked down and stunned, to seeing if you passed out, to seeing that you did and that you'll wake up in X minutes, to seeing how many HP you lose to bleeding during that period, to checking for death for losing too many HP...
Having said that, I have an easier time keeping track of GURPS's rules than DnD 4E. At least almost everything applies to everyone in GURPS, but in DnD 4E you've got so many unrelated abilities that only work in specific circumstances and stack specifically and so on...
GURPS also does suffer a bit from the fact that by attempting to be generic, it doesn't handle any given setting as well as a dedicated system likely could. In particular, it has some fairly big problems with modeling some super hero types, which really boils down to one thing about GURPS that's already been mentioned: it's based heavily in realism. You can tweak it to fit almost any setting, but it takes lots of ham fisting (which is a character trait in GURPS by the way!) to get it to feel like Dungeons and Dragons, for example. Gritty fantasy, historical settings, modern settings and futuristic stuff are all pretty easy to do though.
That said, there are also some definite quirks in just about any genre you apply GURPS to. The default magic system presented looks cool at first glance, but has lots and lots of problems when you analyze it, leaving you probably better off using the built in advantages system instead. Space opera may have problems with the realism that spaceships are grounded in, such as their extreme fragility. If you're playing a pirate game, you may quickly realize that the crew can sink an enemy ship with their hand guns in short order. Things like that.
As a whole though, I'd definitely recommend giving it a shot. Especially with a little house ruling here and there, it can serve almost any purpose.
As an aside, if you're interested in a less complicated generic RPG system, you could check out the D6 system. It's completely free now and can also simulate most settings pretty well. It used to even be used for the Star Wars RPG before it was converted to the D20 system, which is how I fell in love with it.