Aliasalpha: I just couldn't play without analogue driving controls. For everything else I'd probably prefer the PC based control too, what I really need is a keyboard with analogue arrow keys
When playing San Andreas, I use a PS2 controller. On foot, I have my left hand on the controller, with the stick controlling movement (L1 runs, L2 jumps, pressing the stick in crouches). The right hand uses the mouse to aim. When I get into a car the right hand goes to the controller, and from there it's just like on the consoles.
This may sound complex, but it's actually really intuitive, and I can't go back to controller-only after it. Believe it or not, switching between the two methods when climbing into a car actually feels quite immersive, even though there's no reason why it should.
But yes, analogue movement + mouse control is a great combo, and it's a shame it's not more viable in other games (as a rule, you need more than three buttons accessible with the left hand).
Also, as for your "why not just play on Xbox" question - the console GTAs run like shit. Atrocious framerates in all four games, especially San Andreas. The PC ports used to be a reliable sanctuary from this, but sadly the PC version of GTA4 also runs like shit. A shame.