The graphics are much better, but I don't like it as much overall.
The story is better and it's more realistic, but that's sort of a flaw. You don't have any really wild and awesome missions (like using a dirtbike to get into the back of a cargo plane and skydiving out of it like in SA).
You also have MUCH less replay value. Once you're done, you're pretty much done. The hidden packages (pigeons - 200 of them) don't give you anything incrementally and when you finally get all of them you get a helicopter that you can get even without completing the missions. Side missions are ok, but nowhere near as extensive.
Car physics are much better, but some people find it annoying.
I'd say pick it up since overall it's still a great game, but I hope the next game is more like SA was.