Exhaust work (headers on 4 bangers, make sure you torch them up real hot), engine mounts, brake calipers, strut mounts, seat rail bolts, transmission bell housing bolts, gas tanks etc etc.
I don't have a 3/8" impact or air ratchet yet, but so far the Hammerhead hasn't let me down. It's fast (7000rpm vs 300rpm on air ratchets), it's reactionless, it's quiet (compared to some cheapies my boss have at work), it's powerful. The only downside is the bulky head, but you'll get over it.
Just be aware that 200ft/lb is not a lot of torque to break loose fasteners, don't expect miracles. It will break all the 10mm and 12mm if you're not careful.