I have a few air ratchets (harbor freight variety), paid around $10 a piece after coupons.
They are not must have in my opinion but speed up work. At least HF ones are fairly week, so they are good only for driving bolts once they are loosened. Saves a bit of time on tasks where you are loosening/tightening a number of bolts at the same time (say transmission pan) or similar.
Given that they are fairly low torque, they end up torquing below spec, so you can then follow up with torque wrench and torque all the bolts to spec.
Another area where they are useful are hard to reach bolts -- you know the cases where you have only a tiny wiggle room, if you can put an air ratchet on a bolt then, it saves a lot of time. With air ratchet, you always must start the bolt by hand -- incredibly easy to cross thread.