Wow! I was actually able to play for 2 straight hours just now... but I can't say I see the draw here. Roaming around is fun but nothing really happens in the world- just players running around, shopping or killing each other. The online matches take forever to get going. The lobby times are way too long and there's always one person who doesn't mark themselves ready so you'll always end up waiting the entire time.
As for the actual online matches, they're hit or miss. Racing is pretty fun but honestly, real players are just as bad as the AI. The team based missions (whether team vs team, or team vs AI) are easily the best and are really fun... even if people don't know what they're doing. Deathmatch is ok, but given the already shoddy aiming and cover system, it's not exactly great.
What makes me want to quit is how long it takes to make money. You can make about 1-4K per match/mission depending on performance but given how long each one takes, it just doesn't seem worth it. I'm not sure if I'm doing something wrong or if the higher payouts haven't unlocked yet or what.