Amazing and disappointing at the same time. It's "always online" in the sense that you're constantly trying to build up your criminal empire from the start. You can play the game in private sessions, but everything is integrated into the online meta game and if you're not playing Online you'll make a lot less money. You're basically a drug smuggler and you play between Vice City and Mexico (and later Los Santos, Liberty City and San Fierro). The story is ridiculously short for a GTA game, only like 20 missions I think? And while you can do side content with your crew you can't do main story missions with them.
The online content is about the same, new cars all the time. But they also have a lot more of a focus on buying property for businesses/hideouts and upgrades for them.