What were you using to build your themes before?
I've used Starkers to build custom websites from ground up, and it's really good for that purpose. However it'll be too much work to build it into a custom framework for commercial themes. Better to just start from scratch.
I was just experimenting with the idea of using a different framework. I generally work with a framework I've built, using the Skeleton grid framework for the baseline. But I try to experiment with different approaches after each theme is done.