If you enjoy playing online with a bunch of ignorant boneheads, yeah! But seriously, I hated it. I bought it and the only thing they improved was the online performance. I was disappointed, especially since that was after a few years of working on it. The single-player story mode is boring; yeah, you can have three different characters but it's basically the same as GTA IV. Therefore, if you liked GTA IV, (with some slight upgrades) and playing online, go for it.
Yes. The world is huge and has many things to do. From bank heist to mountain bike racing it's a really fun and time consuming game! The story is fun and the multiplayer is awesome. There are so much to do that it'll probably take you a while to fully 100% finish this. This game is a win/win!