I usually do not paint the plastic minis that come with games, but I am beginning to think I'll start.
As whiterook said, I painted up a bunch of the promo aircraft for DOW's Memoir '44 Air Pack, and I was greatly pleased with the result even in such a small scale (for me). Most of my painting has been 25/28mm miniatures for a variety of games, although I did paint 1/72 scale plastic soldiers when I was a kid.
I also started painting some of the minis for DOW's Battlelore game, which I think turned out pretty decent. For me, much of the decision is based on the game itself. Personally, and I am sure many will disagree, medieval and ancient battles were more colorful, and hence are more prone to benefit from painting. By the time of WWII, uniforms and vehicles were more monochromatic and so, IMO, the fact that the figures are unpainted does not matter quite so much.
Anyway, that's my two cents worth. And here are a couple of shots of the planes I painted for the DOW contest. Here is a pic of the lot of them, including my own Lend Lease Russian P40 inspired by whiterook's but in a different paint scheme.
This is the Me-109 that got me the second runner up prize in the DOW contest.
Here is a pic of a Corsair, from which I got my avatar.
And lastly, a Zero lining up a kill on a P40...
