I use Vallejo paints pretty frequently and LOVE them. Hands down my favorite acrylics.
They thin down well with Future, Vallejo's own thinner, or Testors acrylic thinner. They also thin with water, but I've had issues with that, especially through the airbrush.
Coverage is stunningly good. Vallejo brushes better than any other paint I've tried, outside of maybe artist oils. I use it for almost all of my brush painting needs.
Through the airbrush, I've had some mixed results. I've found it sprays well when thinned with Future or Testors acrylic thinner. With Future, the result is a smooth, hard, semi-gloss finish. The downsides, so far as I've found them, are that 1) the Model Color line doesn't reduce very well at all, so I'd still stick with something else for filtering work and 2) I've had problems getting them to spray in fine lines, so I'd be hesitant to use them for freehand camo.
I've also found, to my chagrin, that Microset and Microsol can eat through Future and Future-thinned Vallejo (though admittedly, after vigorous use to try to remove a decal-gone-bad).
Moving forward, I want to move more and more to Vallejo. IMO they're probably the best all-around paints on the market.
Oh...and you can get them through Squadron, Great Models, Sprue Brothers, etc. My LHS stocks Model Color, but intermittently...last I was there the rack was pretty stripped down, no blacks or grays, tons of salmon red and lavender and whatnot.
If you check out the Dauntless in the completed builds section of my blog, it's a Vallejo job. As is the Mossie nightfighter. And the blue nose on the P-51D. And the tracks on the Panzer III.