I got a Harbor Freight detail gun and really love it. It IS of course limited on the area you can really paint at one time.
I shot my roof (54 4-Door) and that was really too much. Notwithstanding, it turned out pretty good. I'd say shooting a door or front fender is about the max size to shoot at once.
For door jambs, floors, trunk, parts, etc. it really is a great setup. Often I mix just a couple ounces of paint for small paint jobs.