Given the amount of players I have and the fact that everyone burns out or needs someone to run ideas by to act as a second opinion/ judgement call, a co-GM is incredibly invaluable. Just make sure it's someone you're comfortable with who shares a similar vision and wants the same thing out of the game, or you may lead to conflict with them. 

Either way, I can't imagine my one game lasting as long as it has without the help of my co-GM.