You thought GTA IV's multiplayer was good? I personally didn't like it. I don't think it will be good until they add a full blown co-op mode. I just think it would be great.
Custom matches, running around with RPGs blowing-up the fuck out of each other with friendly-fire off, running over and shooting prostitutes when they offer you sex, running around knifing random people, ramming into each others cars, driving at high speeds running from swat and the police, camping in the strip club with a fuck-ton of guns, scooter races, and just generally fucking shit the fuck up. I think it was, and still is amazing. If they haven't pulled a Bioware and completely ruined the series (which I doubt they will), I think this multi-player will be just as amazing as the last one.