Here's a report on the usability of the Tor Browser Bundle: https://iscproject.org/Tor_UX_Report_v1.1_05-06-13.pdf

It covers the usability of the Tor website, and it is a very informative read for anyone working on UI design. Parts such as "The download process was cumbersome due to uncertainty around whether to select the 32 or 64 bit version" are helpful to keep in mind.

I hope this link helps everyone.

Will