I use Airborne and Guard almost exclusively.
Airborne is great because it gives you a massive altitude and speed advantage, if used right, and makes reaching objectives so much easier. (Note it makes objective games harder because you get used to your speed, which is hampered so severely when you have an objective.)
Guard is good if you're guarding an objective or your stockpile point. Someone lobs a grenade or 2 your way, just armor lock for half a second, let off the control, and start owning. Note, however, assault teams are often two-man. One armor locker will NOT repel 2 enemy Spartans. However, should you have 2 armor-lockers, you're set.
Tips and tricks:
When using the jetpack, don't just hold down the button. Ascend to a good altitude and airspeed, then use short bursts of jetpack, combined with momentum, to keep you airborne. This yields maximum airtime, and I've crossed Powerhouse on a single airborne trip.
The jetpack is useful for climbing stairways on either side of Sword Base.
On Sword Base, if you fall off a ledge, use the jetpack in a short burst just before hitting the ground to prevent damage.
On Sword Base, get the Plasma Launcher and fly above the ground, locking onto enemies, then firing 2-shot bursts.
Use armor lock in short bursts. You don't get the EMP, but it recharges quicker, and lets you get back into the fight.
The best way to use armor lock is to wait for imminent death, such as grenades lobbed into a room, then lock until the 'nades explode. Unlock and blast the sorry sons of bitches who just rained on your parade.
Also, the default controller setup SUCKS. Use Recon if you liked the Halo 3 setup at all. It remaps the buttons, so B is melee, RB is action, LB is armor ability, and X is grenade selection.