It's a GTA game, many probably already know what to expect. For me personally the single player didn't really take huge steps ever since Vice City or even GTA III. It's pretty much the formula ever since. As always you get an engaging story with interesting characters, loads of main missions, quite a few side missions, plus you can hand out with NPCs to do activities. All in all very good, but nothing revolutionary either.
Then there's the multiplayer, which is the main draw of the game for many people nowadays. So if multiplayer is your thing, you'll have one of the best open-world online experiences you could ask for.