Although anyone thinking about jumping into airbrushing should definitely consult more experienced users than I am, I think that my perspective can still shed some light on the process. Since I just started, the initial trials and tribulations are still fresh in my mind.
So, do you need an airbrush? Use some of my criteria below as a guide:
First, and most importantly, figure out why you want an airbrush. The whole kit, from the airbrush to the compressor to the special parts and paints, will definitely hit your wallet pretty hard. For me, the selling point was speed. Painting is not the most enjoyable part of the hobby for me, so anything that sped up my painting was something I was interested with.
Second, do your research. I am lucky in that I have a good friend who is a Golden Demon-caliber painter, and so I went to him for advice. He recommended that I get one. Then I did the usual internet research: blogs, videos, etc. Then I asked around Brother's Grim Games, my FLGS. A good friend of mine there, who is an outstanding and meticulous painter, highly recommended the airbrush.
Third, and I don't want to sound too much like a free spirit hippy, but are you inspired by something that requires an airbrush? I saw my painted miniatures, and though I am not in love with the painting side of the hobby, I am one competitive dude: I knew that I could do better with the right tool, and put a product on the table that my opponents would respect. I saw that the best results for a lot of camo schemes come from an airbrush. Maybe you see a color scheme that can only be achieved with an airbrush and you absolutely cannot paint without that specific scheme...
(my compressor; came with a free nail gun! I had to buy the filters separately, but....nail gun!)
These three criteria will help answer the "why?" of the airbrush conundrum. Honestly, the question of "what?" (What brand of airbrush should I get? Double or single action? What size compressor?) is beyond me right now. I chose the Badger 105 because they have a great warranty program and their staff seemed super-friendly during my research. I would recommend a double action because if you're going to get wet then you might as well jump in head-first.
Last but not least, if you get an airbrush: use some test models for practice. And then practice some more, and then practice some more, then maybe put a basecoat on some stuff, and then practice some more. Even learning to clean the thing takes practice. But stick with it, because you will absolutely get better.
p.s. Wear a painter's gas-mask. I borrow an industrial mask from my dad's boat and car painting equipment.