I have, and love it. This is the first Grand Theft Auto I've ever enjoyed. Earlier entries in the series just didn't do it for me. The controls were frustrating, and the humor was a bit too sophomoric in my opinion.
Then the reviews came out, and I figured I'd give it another shot. Several things have changed for the better. First, there's a real story that engages you - I'm actually interested in finding out what happens to Niko, and don't see getting bored of the game anytime soon. Controls have been fixed (though driving is still tricky), and multiplayer has been added - and I've had a blast online with friends. The open world has really taken a huge step forward as well. I loved Crackdown, but Liberty City is immense, and really does feel like a living place. Rockstar keeps hinting at an MMO version of GTA, and I can finally see the appeal.
Anyway, I suspect most people have already checked it out if sales are any indication, but if you haven't, because like me, you were turned off by earlier iterations of the series... give it a shot. I suspect you'll be pleasantly surprised!