I use an airbrush frequently for T-shirts etc.... They are a great tool. You aren't doing any lettering so you don't have to worry about consistent line structure, or making skin tones with transparent paint here so its not that difficult.
As far as being able to paint with them if you just think of it as an expensive spray can it is very easy to use. For custom mixing of colors this is by far the way to go. Now if you are talking fiberglass here you can get some nice colors for auto detailing and murals for hoods at most craft stores great for metal and fiberglass. They also take very well to clear coats, also available in the same line of paints. Stay away from the t-shirt paints and you should be fine.
