I use vallejo model colour exclusively at the moment ... I love the matte finish it gives me and it's very easy to blend with. However for my dark elves I will be using gaming colour again
I have an extensive collection of GW paints and have a nice large collection of Vallejo's! I use the GW paints more for basecoats these days and the Vallejo's for blending.
I use whatever I have. Mostly when I'm looking for new colors I pick up VMC, though.
I think it also depends on what I'm doing. I think GW metallics are really good, and I use watered down Boltgun metal for just about everything metallic.
I also like the fact that there are 200 some odd colors for VMC and VGC combined, which is really nice. I hate to mix colors (because I can never replicate the process), so having a wide variety of colors straight from the bottle is good for me.
i used gw paints and have lot of valejo but i prefer the gw paints for their texture more elastic...i only use medium mat of valejo for my very diluted wash