About a year ago, I was fortunate enough to win a complete set of these foundation paints. But even previous to that I had picked up a few colors after hearing a fellow painter talk about them. The Foundation paints are basically just formulated with a very very high pigment ratio. The intention is to provide excellent coverage in as few coats as possible. However, before I get too far into that, let's look at some actual test results:
For my first test I started with a blue/green-ish color painted over a piece of white plasticard. I should point out that I did not prime the plasticard at all. Each stripe was painted using a quick single stroke, with the intention of applying roughly the same amount of paint for each. My aim was to apply an amount of paint that would attempt to cover, but not diminish details too much. From left to right, the paints used are P3 Coal Black, Vallejo Dark Sea Blue, GW Foundation Orkhide Shade, and Reaper Master Series Green Shadow. As you can see, most of these covered quite well. Blue/green provides a good level of coverage in general, particularly in darker tones. On to test number two...
Red is traditionally a difficult color for a number of reasons, not the least of which is coverage. From left to right we have P3 Sanguine Highlight, Vallejo Burnt Cad, GW Foundation Mechrite Red, and Reaper Master Series Violet Red. Comparing these there is little contest. The only reason the Vallejo color does so well is that it's a darker shade (sorry, I didn't have a lighter shade on hand). The P3 color does an ok job, but a second coat would be necessary. The Reaper color does a pretty abysmal job.
So, were these tests fair? Yes and no. To make for a really fair test I'd need to get matching shades, and unfortunately I couldn't really afford (or want) to go out and buy matching shades of paints, particularly if I have no intention of using them again. However the test is pretty fair in that I didn't thin anything, and the target surface was uniform and the application amounts were fairly even.
Would further tests help? You bet. Sampling more colors and testing on black primed surfaces as well would provide other comparisons. I can vouch that the Foundations perform quite well over black primer where almost all other lines require a couple coats, particularly for lighter colors.
What are the limitations of the Foundations? Two big limitations are the relatively limited color range, and the speed with which the dry. I can't emphasize the latter point enough. They dry fast which means you can't do any meaningful blending and you need to pay attention to the build-up in your brush. It also means you need to pay attention to putting down a smooth coat the first time or you could end up with some added "texture" to your model's surface.
Are they worth buying? Yes and no. Depending on the color they range from little more than convenient to absolutely necessary. I've personally gone through more than one pot of the Mechrite Red, and the range of yellows/tans are extremely helpful. Other colors like the blues get nearly no usage. Honestly it boils down to this: If you have colors you commonly base coat with and have to do more than 2 coats, you might consider getting a Foundation color that matches relatively close to your base coat. Also I wouldn't necessarily recommend these for competition work since you need to be careful in applying the coats due to the thickness of the paint.