After using OS X & Ubuntu for quite some time, I never liked the program launcher on top. In OS X it was a bit ok to have it on th top as they have divided the dock, but now in 7 with the Superbar also having certain characteristics of the dock like IE showing progress of download, messenger icon being dynamic etc. I like the taskbar down.
Just my opinion.
THough I know a couple of friends who have the tasbar on top, I just find it a bit difficult to use, maybe I'm used to having it down.