I wanted to get it but I didnt know how the online was.

I've heard that the online is the best part of the game and the only reason to buy it...I only tried the demo and didn't really enjoy SP.

Yes, the online is the best part of this game. Age of Booty is a game that requires a lot of strategy and coordination between teammates to be very successful. In single player this simply leads to frustration because your AI teammates are terrible and there is no way of issuing commands to them so you're usually left trying to do everything on your own. Online however, if you get with a good group of people that are talking this game can be loads of fun.