I've been using the Topo by ErgoDriven. I like it, but at the same time, I'd like to try something that isn't as soft. I should add that I prefer to be barefoot whenever possible and when I'm not I'm usually wearing shoes that don't have any support or much cushioning. I can't think of a natural surface that I've been on barefoot that is remotely as spongy as the Topo.