Ach, what can I say? Immense, completely immense. Didn't think Niko was going to be such a likable character at first, but he's brilliant (not as good as CJ, though :P). Brucie's got to be the best character ever in the series.
The missions are great, the taxi service is brilliant, finally the tedium of driving cross-city is over, and there's nothing better than running into a guy, making him retaliate and having HIM arrested. Brilliant!
The funniest thing about GTA:SA was fat CJ, so it's a shame you can't get fat Niko. But hey, when you can play online against 7 friends, all in helicopters, fighting police copters, you tend to forgive the little things. :D