Ubisoft has put forth a video showing about 9 minutes of Watch Dogs' multiplayer aspect. The game actually seamlessly connects the single player and multiplayer game in a fun way. You can see the various ways you can get into multiplayer with a friend or friends in the video.
I really love how your friends can appear as NPCs in your game and you can initiate a multiplayer game with them. You don't have to get into some other mode to do so and it's this smooth transition from single player to multiplayer and vice versa that has peaked my interest.
Both team and single player online portions are shown and each look fun in its own way. I'm really excited for this game to come out and I can't wait to get my hands on it.