I've been frustrated and unmotivated in Matchmaking lately, so decided to give Firefight another shot, and holy crap. So awesome.
It's especially good in local co-op. I babysit two brothers (10 and 12 years old), and despite their vast love for Halo, they're not so good at it. So, to make it fun for them while also a good challenge for me, we play Versus Firefight. Me as a Spartan, them as Elites, difficulty on Heroic. They totally destroy me, and we're all grinning the entire time.
The cooperative gametypes are great, too. We've had some amazing runs in Firefight Classic and Generator Defense. The sense of survival and the necessity of team work make it an awesome experience.