This game is pure genius. The immensity of the environments, the awesome variety of gameplay and the improved mechanics make this game vastly better than the previous ones in the series.
Plus, you can swim.
Load up Vice City after playing San Andreas for a while and you'll instanly notice the difference in graphics. Vice City's models look extremely low rez in comparison, plus the screen looks somehow blurrier in VC.
And in VC you can't swim.
The story is excellent, with a huge cast of supporting characters who are well played. Much of it is humorous - Wu Zi Mu's missions in San Fierro are a hoot - but when it wants to be serious it can be. Plus, it's long. The core missions alone can easily last you forty hours, not withstanding some very entertaining side missions. There's only one series of missions in the game that I had a real complaint about: Zero's missions in San Fierro. Luckily, they're easily skippable.
Plus, you now have statistics for everything, including fat, muscle, stamina, and even weapon and driving skills which change according to how you play the game. It even changes how people on the street respond to you - and you can respond back!
This game is generally just fun to wander around in, especially once you get ahold of one particularly entertaining air vehicle.
The one weakness in the game is that the radio stations aren't nearly as thematic or funny as the ones in Vice City. But that's a small price to pay for the myriad improvements in the game.