I used to play GURPS a lot. I'm an edition behind, but I don't think the fundamentals have changed all that much.
GURPS, as others have remarked, is almost a lot like a toolbox than a completed game. It doesn't really play "out of the box" all that well. You need to decide what options you want to use and so forth. It also has fundamentally different genre assumptions than D&D does (which are, admittedly, sort of nutty). D&D has a sort of exponential power curve where a 10th level character can just ignore 20 1st level ones (roughly, you know what I mean). GURPS isn't like that. And, if memory serves, it's going to be hard to have like 4 combats a day without some serious magical healing or use of the so-called "cinematic" rules. So, it's a lot grittier: more Game of Thrones and Robert E. Howard's Conan than D&D.
That being said, it is very well thought-out system. And, it has a very nice internal logic. For example, Veekie mentioned the magic system, and it's very cool: essentially it's all skill-based, with each spell being its own skill (so you can be really good at firejet but not so good at fireball if you like) and casting wears you out.
The only drag I'd say is that it does have a lot of situational modifiers or permutations, which can be a headache, especially when you're getting started. Like any complicated game (D&D included), it's nice if you start slow and if everyone pitches in. There was a decent Fantasy Sourcebook, though I didn't like their setting for it (Yrth, iirc). I did love their Conan sourcebook, though. And, overall, their sourcebooks are pretty brilliant.
But, overall, it's a system best suited for more skill- and adventure-based games that are grittier than D&D. It's got a lot of flexibility to it, so you can make it do what you like, but then you're not playing to its strengths. For example, you could play superheroes in GURPS, they have a whole book for it and everything, but I'd play M&M instead.