I never played the missions on San Andreas. I just messed around, blowing up stuff, running away with whatever I could find. Once, I even made a group of police officers run after me on foot, then I killed them and hijacked their car. It was sweet!!! =)
The only thing I don't like about the customizer on San Andreas is that you can't see how something will look before you buy it. Sometimes I'd spend all my money finding the right rims for one car.