I bought an airbrush late last year, its a Paasche Single Action External Mix (one of their cheaper models). Honestly, I am kicking myself that I didnt get one sooner. I haven't looked back since I got it. I haven't done any fine detail work with it (yet) all I have used it for is for base coating.
Just over the last few days I have finished putting together 7 Marauders and, after undercoating them with GW Chaos Black Spray, I base coated them with the airgun and it took all of 15 minutes for the first, then an hour of drying time before the second base coat, only another 15 mins. All of this would've taken me days with a brush.
I did a similar thing with my 30 odd MI Powersuits Troopers. Several days work done in about 20 - 30 minutes....awesome. The finish is also smoother than what you get with a brush.
Without a doubt one of the best purchases for my modelling hobby I have ever made.
I have not had a lot of experience with these things though but one thing I will say is this. When I first got it I bought a can of compressed air to power the thing.......what a load of crap waste of money that can was. It didn't provide enough force to fully mist the paint so it came out in spurts and splutters.......I flew into a rage and through the can across the backyard because it wrecked a model. Buy an air compressor...even a cheap ass one from the hardware store. I bought a oiless compressor made in China for the price of counter meal at the pub and it is perfect for the job.