Note that I am sweating and frowning. This is very important, as it sums up how I feel about the Quad Mill.
I pretty much hate it.
You do this squat thing and the platform you're standing on moves up and down while you maintain the same head level, using your legs to compensate the change in height. It's a very good exercise, as far as benefits go, but it rates an "above average" in the "pieces of gym equipment that make me want to cry" category.
Consider yourself arch-nemesied, Quad Mill!