Now is the time to send a message to Ubisoft that this sort of intrusive DRM will NOT be tolerated. If the servers had stayed up and people just refused to buy the game, they would have written the poor sales off as being caused by "pirates." Now, you have a chance to prove otherwise. Every single person who bought this game on PC should return it to the store. Yes, most will attempt to deny the returns due to policy, or to exchange with a new copy since that one is perceived as damaged/defective. Do not stand for this. Tell them that yes, it is defective, but ALL copies are defective. Let them know that the software itself works just fine on your computer, and in fact ran EXACTLY the way it was supposed to. However, you are forced to return it because it does not work properly on yours or ANY system, because Ubisoft's servers weren't online to allow you to play a game that you legally purchased and met all the requirements for being able to play.
Ubisoft won't be able to shrug it off as "piracy" when their sales numbers for this game begin to shrink due to returns and angry retailers. THIS will hit them in the pocketbook more than a simple, dubiously effective boycott. When they are forced to start handing money back because of their failures, that will speak much louder than never having been paid that money to begin with.