I'm not a huge fan of Gurps, and I'll go into why for a bit. Keep in mind that I've only actually played a coupla times, and thus, am not an expert in it's rules.

1. It's kinda a meta-system. More of a D20 analog than a D&D comparison, if you get me. IMO, meta-systems are clumsy, require a *lot* more work to build a good setting from, and in general, making it harder to have a good, believable setting makes it harder to have a good game. On the flip side...it's not the worst meta-system I've played with. I had the misfortune to try Hero once, back before I finished post-calculus math.

2. I prefer blended systems rather than more consistent systems. I love than in D&D, the casting mechanics are interestingly different than psionics, truenaming, straight melee, or what have you. As you've probably guessed, Im referring to 3.5 and previous, as 4.0 is a departure from that. From what I've seen, Gurps doesn't have the same variety in mechanics. Now sure, this makes balancing the game easier, but frankly...I don't care.

3. As for customization...sure, there are a great deal of options, but who would really say that say, D&D 3.5 lacks options for characters? Most of the RPGs I play anyhow are great on customizability, so that's really not a big draw. It seemed like a generally slow game to play, too...but that may be because only one of us was familiar with the system. Hard to call on that one.

Eh, depends what you're after, I guess.