I am looking at getting an airbrush machine for christmas. Are they worth it and which brand do you suggest? I have found some on Amazon that are fairly inexpensive, but I don't want to waist my money.

I suggest if you just start out learning, go cheap.
It is an art form to airbrush on a cake. When you get better you have also a better idea what you really need, how much you will use it, and what price is justified for your needs. I found a compressor and airbrush used for under $100 and it works great for what i use it for.
good luck, airbrushing is great fun and does awesome effects on your cake.
ChrisKrumbs