Halo and Halo2 are the ONLY M-rated games I let my kids play. I disagree with the M-rating for those two games. Sure, there's a lot of gunfire and explosions, but that's the fun of the game. Then there's a little bit of neon blue/green alien blood when you kill a baddie. Kind of cartoonish, so OK. If a human gets killed, there's a little spray of red on the ground or wall. Not bad.
I'm VERY selective about what TV shows and movies my kids watch and very selective about the games they play. Halo and Halo2 passed the test and my 9-year-old daughter and her 10-year-old brother finished Halo2 in about 3 days. I watched them do most of it. Besides the limited blood and a little bit of PG-rated language, the game was just fine.
If it was rated like a movie, I would give it a light PG-13 and I've let my kids watch a handful of those depending upon the content. R-rated movies are not permitted nor would a game that would earn an R-rating. Halo and Halo2 would not, in my opinion.
If you don't believe me, play it yourself for a while. Buy it from a store that would let you sell it back if you don't like it. Sure, you won't get what you paid for it but at least all your money wouldn't be wasted. My bet is that you'll approve it for your kids to play.
Are you familiar with CounterStrike? How does it compare? I guess as long as they're killing aliens -- rather than other human beings, as in CS -- Halo should be okay.