It's fun to play with and look at, and I feel with some refinements, it COULD be useful, but there's two big issues I have:
1) New files appear on the desktop right in the center by default, versus the corner. It's fine if you have NO Finder windows open, but odds are it's going to require you to move it.
2) Opening any folder or window just switches to the Finder anyway, so unless you leave everything on your Desktop, its usefulness is quite limited. If there was a way to stay inside BumpTop and use its interface to browse more than the Desktop, then they might have something.
Besides all that, the stacks are pretty cool, and I do like the variable icon sizes quite a bit.