I tested all the main keying software (Primatte, Ultimatte, Zmatte, etc) for my company and decided to go with Ultimatte.
Ultimatte has some definite advantages over the FCP Chroma keyer, namely in hair and edging. In the hair you are easily and quickly able to achieve definition in someone’s hair without the noise and wierdness you sometimes get using the FCP Chroma key (especially dealing with the uncontrollable metal-hair I was looking at). Also in situations where the lighting wasnt that good the results you get with Ultimatte are pretty impressive.
Another plus over the other keying programs (at least at the time when i tested them, June 09) Ultimatte takes advantage of all your processors so render time is kept to a minimum (of course there are other factors that play into render time- codec, other filters, etc).
So long story short, I think you will be very impressed by the results you get with Ulitmatte, but I would download and play with the demo version to make sure you like it.
-Dave
LTM