DV Garages DV Matte Pro is by far the best choice after Shake and After Effects. Way better than FCP and even better than Motion! Plus, it's the fastest thing on the planet for pulling keys.
Try KeyLight in After Effects CS3 if you do have one. I don't use FCP for keying although it should give good results if the footage was shot properly against a green screen (proper lighting, camera color sampling 4:2:2, etc).