Hi, Rob,
I have not used any of Vallejo's sets, but I have bought individual bottles, as I have been working more frequently with acrylics. Generally, I find Vallejo to be of high quality, that is, the paint goes on very well, covers well, and a little goes a very long way. I use water to thin them. The only issue I've had so far has been with their Prussian blue; it dries with a semi-gloss sheen, that I have not been able to make matte. Andrea's Prussian blue dries flat, though, so I use the Vallejo on my toy soldiers instead.
I haven't bought any Vallejo sets, yet, because I can buy enough individual bottles to get the same results, for less than what Vallejo charges. But I recognize that many painters and modelers prefer not to have to spend time with that kind of color selection. It's a similar situation with pre-mixed weathering powders or washes, like those sold by AK Interactive and Miguel Jimenez. I can grind my own chalks and make my own washes, but I understand that AK and Mig take the guesswork out of it.
As far as application goes, I'll place a drop of the paint on my palette, and then mix either 50-50 paint to water, or even thinner, if I'm laying on a glaze coat to build shadows and highlights.
I can definitely recommend Vallejo's colors. I haven't tried the airbrush range, though.
Hope that helps, prosit!
Brad