Never played the multiplayer myself, and as everyone has pretty much said, single player was boring very quick and not much to do in the game. There were far too little missions that were actually fun or exciting. The odd cruise around the city aimlessly whilst enjoying the graphics isn't enough to warrant the money spent on it. Never finished the game and had no interest in doing so after a while.
I traded in SA ages ago, and pretty much regretted it since. That's actually the reason I bought IV. I think everyone needs to have at least one GTA in their collection, for that mood that only GTA can quench.
Hopefully this next one can combine the great graphics with great gameplay, more things to do, and spend money on.