I tried Vallejo Xpress colors a while back (also on my Urbie lance, coincidentally) and I'm not a fan of that type of paint. Contrast/Speed/Xpress paints work well for me on organic (fantasy) minis with fur and scales, and I also like the effect on BT infantry. But the big flat panels on mechs don't play to their strengths. They're OK if I'm in a hurry, but I can do better without too much extra effort.
What I *do* like is using the fact that they're translucent, thinning them down and airbrushing them over a pre-highlighted grey base/primer coat. That does take more than 30 minutes/mini, though, so you might be on to something.