Just got off 90 minutes of co op. I was playing by myself, got a message saying someone wanted to join, said yes, and he appeared like 15 seconds later with no hitch on my side. We did some side missions. First was escort, where you have to drive while a hooker fucks a guy and keep the press from taking pictures. Well, in co op, one guy drives and the other guy has to do a sex minigame. It's awesome. Then I got wanted stars but didn't want to pull out the map and get arrested, so he set his own wavepoint and talked me to a Forgive and Forget.
Then we did the demo derby. We completed it, raking in like 8 grand, earning 15% off all vehicle modifications at the shop, and unlocking the full set of derby cars at our garages. We also unlocked all the demo derby upgrades, like cattle catchers and boarded up windows.
Then we went clothes shopping

Then he quit, which was also completely seamless.
This game fucks GTA4 in the ass.
edit: Oh, and I upgraded my apartment with a big screen TV and a stripper pole, and it changed in realtime on his screen. We played some of the Zombie attack game, but lost.