I ended up buying an airbrush last year. I got one of the detail brushes (Harder & Steenbeck 0.2mm) and it's really handy for some things but I just don't have the skill to use it to its fullest. I find it saves me a lot of time for painting things like 15mm armour and 1:300 scale ships but for doing things like 28mm figures, I don't get much use out of it.
If you're mainly going to use it for priming, it doesn't save a lot of time but it does give you a much better coat. It's also great for adding zenithal highlights which lessens the need for shading.
Make sure you get one with a larger needle (0.4mm or more) as the fine ones (0.2mm or less) won't give you the coverage you need for quick priming.
The Paasche Talon I've been grokking comes with a 0.38mm, which appears to be a good middle-road size for doing primer, base coats, and zenithal highlights.
I'd still rather use brushes for the rest, so hopefully that would be good enough.
Although there's another kit, for that same airbrush, which also includes two more needles both smaller and larger than that (0.2 and 0.5 ?). I suppose I can just buy a larger needle, individually, if I ever need to put paint on something larger.