I'm currently in the middle of doing a Halo 3 Marine helmet for a friend of mine that requested it for his birthday. The Marine helmet is pretty nice and I definitely agree that it's an underrated armor set. Especially for the mobility aspect. It's also probably the "easiest" for doing an undersuit, since you can use BDUs/cammies from a surplus store.
If he's really nice, I'll probably do the rest of the armor for him as well and then use that peer pressure to turn him into my handler at local Cons.
Helmet is looking good, keep us updated on the progress!