OK, my 0.02 here, as someone who played GURPS 3e for 15+ years (only reason I'm not playing now is lack of a group) AND owns most of the WotC d20 books except for the setting-specific FR and Eberron stuff.
GURPS tries to be "universal", but I suspect that's something that may not be possible for ANY game system. Similar to Goedel's theorem that any "language" (including mathematics) will have at least one possible valid "sentence" (equation, whatever) that doesn't "parse". (e.g. "This sentence is false." is neither false nor true in English, despite being "valid" - following all the rules for sentence structure.), IMO any game system is going to have at least one setting it doesn't simulate well.
GURPS works very well for grim-and-gritty, low-magic campaigns. X-Files? Saving Private Ryan? King Arthur? Cthulhu? Constantine? Dracula? Firefly? Babylon 5? Sam Spade? All good.
Star Wars? Star Trek? Lord of the Rings? Battletech? Warhammer? Batman? D&D? Sin City? Cyberpunk? Not quite so good, but doable.
X-Men? Thor? Dr. Strange? Superman? The Matrix? Wuxia? Slayers? Not really up to it.
Generally, the farther away from "standard human" a character's race/skills/powers are, the harder it is to simulate him in GURPS. The system handles swinging a sword or shooting a pistol quite well, but doesn't scale well to swinging a battleship or shooting a Wave Motion Gun.
What I really like about GURPS is the level of research and playtesting that goes into it, which is one of the reasons the supplements are so good. Subscribers to the Pyramid boards can preview, playtest, and suggest edits for upcoming books - I've got my name on the credit pages of a couple of supplements that way. This allows the supplements to take advantage of the collective knowledge base. In the Space playtests, for example, we had a couple of posters who were actual astronomers and physicists in their day jobs. This tends to weed out the most broken/unrealistic stuff. Not that there AREN'T broken combos - but they tend to come about when you combine stuff from different books/genres. This is also probably the reason GURPS works better with more "realistic" genres. If something has real-world stats (e.g., fuel consumption of an M60 tank), somebody will probably post it and the book will be tweaked accordingly. But there are no real-world sentient AI programs, or people who can lift a battleship, so any stats on such abilities are purely off-the-top-of-the-head and cannot be fact-checked.
In my experience, playing a GURPS character doesn't require any more math than playing a D&D character - both tend to require summing up a lot of modifiers. Creating a GURPS character is a little more complex - but more flexible, and for the most part, it's a one-time cost: you do the calculation, write the resulting number on your sheet, and just use that number from then on, no need to redo the calculation each time. It's the GM who needs a calculator when designing adventures (but much less so when running them).
I play both, I like both. Which is better, a knife or a spoon? Depends on whether you're eating steak or ice cream.