wow a question i can answer!
Right firstly, Pros:
they are easy to use, and basic airbrushes can picked very cheap, about £10, the one i use is about £50, and is quite a good airbrush (a Badger 200, if you are intrested)
They can also spray nearly everything you stick in them, as long as you thing it to about the viscossity of milk (semi-skimmed of course)
Can be expensive
You need something to act as a proppelent, there are two options: a Compressor (expensive, but worth it, constant supply of air) or airbrush aerosol cans (cheap, but not much air, u would probably use about 2