FF is a single-player mod for Doom3 aiming to bring the classic gameplay to the maximum. Lots of action, 12+ new huge maps, new monsters, new weapons, new soundtrack, advanced AI, power-ups and a well-balanced gameplay that will push your skills to their limit. Add the ability to drive a vehicle and to fight as a demon in 3rd-person perspective and you get a good idea of what is waiting for you :D
Wow, this enhances RoE/Doom3 in breathtaking ways.
So except for a few problems with controlling/fighting as a demon (it's too jumpy, ouch), it's pure awesomeness.