Finished it. It's a great game, doesn't have the low points of Halo 3, but none of the really heigh points either: getting on scarabs, really tough up and close brute fights in sierra's nest, more sandboxy vehicle gameplay. The story wasn't really clear to me until it started tying into the first Halo.
Fighting elites is awesome though and the feeling of being in a war is at times really well developed but the armor abilities are a bit underused (mostly it's just running or one of the defensive abilities, so nothing offensive). Wish there were more points where I could use a jet pack: it's so much fun.