I like the online because it lets you create your own character, too bad they are mute. The SP portion has some great story elements and it's not a bad part of the game, but I have more fun playing freeroam with friends than alone once the main game was over.
The only thing that could have kept me on singleplayer was being able to do police missions, ambulance missions and even firefighter missions.
But they took them out, so the mode is ruined for me. If it weren't for online I would have returned the game already, outrunning cops gets stale fast... and the fact that everything you do makes it so you get into a cop shootout or a run is not fun. I mean, if you stand too close to NPC's they call the cops and you have to run away.... pathetic.