I ran across the results of the Debian Popularity Contest today. It doesn't claim to be anything of a scientific survey , just the results from people who voluntarily run a script on their own machine. As of today, Bookcase is tied for 4484th place by the number of people who use it regularly and tied for 6544th place among those who just have it installed.
And those stats are for version 0.6.6. I hope they can get it updated to 0.8 soon.