Me too - it feels really quite shallow. There's some nice looking graphics in there, but not a great deal of actual gameplay. I've played 4 or 5 times, and not bothered after that. It's a shame, as I played GTA and GTA III through to completion.
Gone back to playing SG and Katamari on 360 now - far more gameplay in either of those
I'd give GTA possibly a 6 or 7 out of ten. Nowhere near perfect.
D.