I agree with Richard. I have heard of few problems with P2 card where as the firestore is dependent on the things Richard mentioned as well as a good cable. I had some friends lose a whole shoot because of a bad cable. People who have had trouble free firestores love them though.
I agree with everyone else so far. The firestore was viable when they first came out and P2 cards were only 4GB. Today with 32GB cards and in the future with 64 and 128GB cards announced there is no longer a need to have to put up with more external bulk to pack around and connections to worry about.
"thou can not stir a flower without crumbling a star" ......Henry Wadsworth Longfellow