Finn isn't useless. There are a number of actions he takes in the movie that lead to him saving lives. The character is comical at times but he still has depth and moments of heroism.
Finn helps Poe escape The First Order, supplies The Resistance with information about The Starkiller weapon and goes back to rescue Rey. Finn takes on Kylo Ren in a lightsabre duel despite not having force powers.
I knew about the lightsaber fight, but I didn't know about the other stuff. This info you provided me has eased my fears about the character.
Rey, Finn and Poe are the big three good guy leads of the new Star Wars trilogy.
This is why I have been telling people for a while. People tend to forget that the original trilogy did not solely revolve around Luke's heroism, but instead revolved around the heroic adventures of a team of rebels. The original SW movies was the perfect example of how to do a "heroic team" movie (the team being Luke,Han,Leia,Chewie,C3PO,R2,and Lando).