Hi Philip, It's very much personal preference - I love Vallejo - If you were at the IPMS Nationals you'd have seen me staggering out under a BIG box full :-)
I also like Citadel paints from Games Workshop which are expensive, but also work well as airbrush paint. I don't like Tamiya, so I'm going against the last couple of posters - sorry Guys
I find they are very grainy paints - I love them for painting details, but they don't like my airbrush technique...
I also don't like Humbrol acrylics, they are even grainier - but I do like the brick red, and the orange for doing rust, as the grain of the pigment makes good texture then.
I think it's personal preference, but I do see a trend among modellers for Vallejo for spraying here!!!