Watch Dogs broke a bunch of pre-order records so it's not too surprising that the game is experiencing problems now that it's out in the wild. Some users are reporting extended waiting periods trying to login but Ubisoft is aware of the issue, the game's official Twitter account commented:
We are experiencing issues with the authentication services. Players may experience long delays when trying to login in-game. More to come.
— Watch Dogs (@watchdogsgame) May 27, 2014
MMOs and online games often experience problems like this after launch, SimCity and Diablo 3 spring to mind, but that's scant consolation for those trying to play the game.
While these issues persist, players will not be able to interact with the ctOS app or engage in player-to-player hacking. Ubisoft expects Watch Dogs to be their best selling new IP in history so that can probably expect a lot of demand on their servers for the forseeable future.
You can check out our review of the game later today.