One shop where I work has an Ingersoll 231 that has been about beat to death, still works fine.
My brother in law has been happily using his 231 at work for five years.
I bought one a year ago that I use every day. It has the right balance of power for me. Strong enough to bust about everything I normally do but not so strong that it breaks stuff.
A co-worker had a THundergun for a while. It would certainly knock bolts off but it was too loud, a bit heavy, and a bit stronger than necessary for regular automotive service. I tried it for a while and that's what I thought about it.
A plus to a 231 (and some other Ingersolls) is that you can buy "tune up kits" for them that replace the internals that wear should you use it long enough.
Beware the 231's with rubber handles. They are more comfortable than the bare metal ones but the rubber softens a bit with repeated exposure to oil. This causes it to loosen up on the gun. You can still use the gun normally, it's just annoying. Most 231's don't have the rubber handles though.