I like GURPS a lot - in theory. It's gritty, it's detailed, it allows for complex character concepts.
In practice, I have yet to play in a really enjoyable GURPS game. This is largely because it's, in my experience (and I've tried a lot of games) almost impossible to find players and GMs who understand the system well, and want to learn all the "optional rules". As a result, most things are resolved with the basic ruleset, which is - as someone pointed out earlier - dry, dreary and simplistic, with no flavor or character.
Also, I find that most GMs, rather than accepting how deadly GURPS combat is (and the fact that it should be used as a last resort) simply find ways to fudge around it - which means that a lot of the time, I was sitting there rolling my eyes at the fact we were still alive when I could think of twelve different ways to kill the party if the enemies had been played competently using the complete combat rule set.
(I don't mean this as a personal attack - and I realize that there might have been circumstances I'm not aware of - but on the surface, reading about something like a combat between PCs and multiple ogres sort of makes me cringe. The fact there were no multiple fatalties as a result of fighting extremely strong monsters so tough they could be hit multiple times and keep on making their Health rolls to avoid passing out or dying tells me either punches were pulled, or the PCs were godlike. Four brutes that use even rudimentary tactics and can shrug off tons damage = let's all full-out attack one PC at the same time since we won't be able to dodge their blows anyway, so what do we need an active defense for = one dead PC)
While I suppose it's possible this could all be a huge coincidence and bad luck on my part, I believe that GURPS actually encourages the above - people strip out the complexity and deadliness beacuse it's too much for them, and you're left with a simplistic system without flavor.
Which also makes complete GURPS one of those settings that's absolutely murderous (for either the PCs or NPCs) if someone on one side of the table is significantly better at gaming the system, playing the game, roll-playing, whatever you want to call it. In D&D, if you don't plan your tactics well, you take an AOO. In GURPS, if you don't plan your tactics well, the first hit you take puts you out of the fight.