I have a lot of air tools but charging the big compressor takes a while, its noisy, you have to keep up the pressure to be effective, and air tools tend to be large and harder to get into spots. Air compressor also takes a bit of cleanup with the hose, and need to empty any water in the tank every once in a while. Oh and remember to oil your tools or they wont last long.

Its also risky. If your not careful you could strip a thread or something.

I only use mine when I really have to, which is rare. Almost never under the hood.

Then again its a life saver when dealing with nuts/bolts that are hard to force loose. High quality air tools will force pretty much anything free. IMHO its a good investment that you will find use for later (nail gun, painting car, filling your tires, ect..) any serious diy mechanic should have one.