From seeing my housemate playing Halo 3 it was a shallow experience. Games rarely last more than 10 minutes before the next map loads and its a different mode of play again with random people. I asked him if you could just play a CTF map until you got bored but it seems you couldn't
The CoD system on the consoles is way better than the Halo one, but it is a similar thing to be sure.
The difference being on CoD you can actually chose your gametype.