It isn't as filled with things to see and do as San Andreas. The land area is much bigger, but you'll see the same things time and time again. There's small villages, military bases, cars, people, some "collect missions" and... not much else.
You've got a huge island chain, some fun stunts, lots of explosions, about fifteen story missions and a ton of repetitive side quests. It isn't as replayable as San Andreas, nor is the story anywhere near as good.
I still had fun with it, at least compared to the demo it was a lot more of a game.