I was very undecided at first. It only just turned up today, thanks to UK post and all.
I put it in, and was underwhelmed at very first. It's the first GTA I've played on console properly (I played VC at my brother's on PS2, but I didn't have a PS2 and got it on PC), and so the graphics and controls felt odd. I always used a dual shock to drive with on PC at least...
That said, after half an hour driving around and seeing the sights, and the first bits of what it has to offer, I am won over. It is massive! I can't believe how long it took me to see most of Los Santos alone!
It does seem more difficult than 3 or VC, but I don't think that's a bad thing, as the penalties for dying aren't that bad.
I think I'm enjoying it more than the previous ones, just because of the scale and scope of it. I will pick it up on PC when it comes out, seeing as until I got this I still went back and played VC as a 'pick up and play' game.