It would only really act as an engine for your game - you'd probably have to add your own weapons and monsters, but you can certainly design your levels and game with FPSC then use DBPro to make it into your own unique game later. I'm not saying it's easy, but it's a good way to cut out a lot of the headaches involved in writing FPS games - heck I think they've even included the multiplayer stuff too.
You would probably be best to get a test level together and load it into the FPSC engine source - then mess around, see what can be done - like you have the bones of a FPS game, you have to add the muscle.
Van-B
Put those fiery biscuits away!