Well, I went out and bough the D&D 4th Edition player's handbook last week. I've actually been playing this with my gaming group basically since it came out, but I kept putting off getting the book, mainly because I wasn't sure it was worth the cost ($35)...
I'm not a huge D&D fan, never have been. I've always thought it was a little silly and unrealistic. A few examples that pop into mind from 2nd edition (which I played a lot of during college) - one minute combat rounds (how many fights do you think actually last more than a minute?) and the ridiculous damage system. I mean, last time I checked, you could be killed by a dagger with one hit - no matter who you are. It's obviously a deadly weapon. However, in D&D it would take lots of hits to kill a high-level character. Combine that with one minute combat rounds and you get some silliness. Like spending half an hour knifing someone before they die.
I could go on, but that's not the point. Back to D&D 4th ed. A lot of people have complained about how heavily D&D 4th edition is influenced by MMO computer game mechanics. That's certainly true, and pretty obvious to me, since I play a lot of those. But I'm mixed on whether this is a bad thing. After all, I wasn't a big fan of the old mechanics. I like that they're making combat more structured, and essentially forcing people to use miniatures. Of course much of that came in with 3rd ed. And I sort of like the new classes/powers, etc. It feels a lot more &"user friendly&" than 3rd edition's giant table of feats and spells and whatnot.
Of course the hit point mechanic is still there, which I don't like. Combat is still very abstracted from reality. How can a &"power&" miss and still do damage? They don't even bother to explain. Which I guess they pretty much have to do if they're going to use MMO mechanics in a tabletop game.
I do like that it's now more possible to build some of the kind of characters I always like - the nimble melee fighter who relies on dexterity rather than sheer strength. That was a difficult character to make effective before - but now a rogue can often add their dex bonus into damage, which is a lot better.
I'm still pretty new to 4th edition - heck I was really new to third edition. In fact, I basically bought the v3.5 player's book about three months before 4th ed. was announced. Which was annoying. But given Wizards' propensity for milking their player base for all they're worth, it wasn't surprising. So I've really only player several times so far, but I'm looking forward to more.
But I'd still prefer GURPS any day!