Figured I should get myself one of these sketchbooks too. Been spending too much time coding lately, and not enough painting. All sketches you'll see here have been painted with Harry Storbacka's Digital Artist, available from http://www.da-gfx.com.
The above image is using two sets of vanishing points, one for the building and one for the spaceship (or whatever that flying thingy is).
Was just doing some testing of the software and this came out (in about 20 min). I noticed the need for a separate line drawing tool for drawing straight lines with two vanishing points selected. It'd be much needed for the diagonals on that fence to the right. I think a separate tool will be better than a 3-key combo with two VP's+shift for lines.
The -20C temperatures here might have inspired this wintery painting.