Here's why you shouldn't get a membership with Fitness First:
Contract states that, with documentation, you will be allowed to exit if your work forces you to move outside of Germany. This is even reiterated to you when you sign up. Sad fact is (quoting directly from an email sent by the Fitness First Customer Service team): "Please remember that it is a goodwill from Fitness First to let you out of the contract before it ends. According to German law, we do not have to do this."
I know a few people who were let out of contracts without any issue when they had to leave Germany. Unfortunately, I and others have recently been having issues with Fitness First charging for several months after documentation and notice of cancellation were provided. Unless you monitor your bank account regularly, they will just keep charging you even after you move.
I know it's a nice gym and all, but the headache of ending a membership isn't worth any of the benefits that FF provide.