What's amazing to me is that this is actually the third incarnation of Quay in almost as many months. Rainer's good enough (and confident enough) that he is not afraid to start from scratch if he sees a much better way of doing things. Quay is designed to work well, yet be unobtrusive.
I haven't been of much help to him testing Quay because I never really used the Dock (my Dock is auto-hidden) in Tiger always preferring Quicksilver. So I didn't miss anything about the existing Dock functionality when Leopard. I did try out the previous version of Quay but... well... it just doesn't occur to me to reach for the Dock very often, I'm conditioned to hit Cmd+Space (my Quicksilver shortcut) if I need to find something.
However from the screenshots I think the new version of Quay looks very stylish. Rainer seems to have put a lot of effort into polishing this version and I think it will show. If you do use the Dock a lot and, especially, if you found the changes in Leopard less than a perfect trade-off, I think you might want to check out Quay after he releases 1.1.