I would never play FPS shooters on the PS3, i only use them for football or racing games, or the odd arcade game, u just cant play these games without a mouse in my opinion.
I use the FragFX by SplitFish Gameware for FPS on my PS3 and it works as well as a keyboard and mouse. It definitely gives me a great advantage in terms of control and precision as compared to the standard Sixaxis PS3 controller.