Seemingly to start with, it is quite similar to GTAIV/RDR with added bonuses. BUT - this takes place in San Andreas, which instantly makes it awesome.
I didn't play too much of GTAIV online - but I didn't play much GTAIV at all after the campaign. The most fun things to do, planes, skydiving, dirt-bikes, riding off mountains etc... were not really in IV, so I can see the online being far more fun, just because of that. It is great on your own, so I imagine that will only improve roaming around with other people (and then into the game modes).
Beyond that - I really hope what they add pushes the boundaries. Heists could be excellent, and the content creator could be truly amazing - but we will see just how far it will go.