Everyone seems to think the AR is useless, but it's actually quite effective if you use it right. The key is to use it in bursts, or if you prefer to spray and pray, then move in close and to the side to avoid your opponent's fire. Also, it seems to work well from a jet pack, probably because precision is less necessary.
By the way, the double melee is a great way to kill someone at close range... unlike Halo 3, it's hard for someone to kill you with anything but a headshot before you can melee them the second time. Also, a precisely timed melee can now counter a sword strike. Shotgun still pwns.
And there's nothing quite like sprinting up behind a jetpacker just as he's taking off and giving him a good smackdown assassination.
-m