I thought it was very good. The story was barely there, but I wasn't expecting much by that time anyway. The atmosphere was great, the levels were great. The music warmed my cold heart with a cool island song. It was neat to see Halo from a different, more 'down to earth' and human perspective. Collecting the audio files was a fun distraction and added a lot of character. Firefight seemed very well put together and thought out. It did not feel tacked on.
Now for my complaints: the price was steep. And as great as Firefight was, this was offset by the absolutely grueling pace. It took way too long to accomplish your goals. You could play for hours with no end in sight.