Bought the add-on to read some large drm-free mobi files; it refused to import them.
Used Calibre to convert them to epub format; ShuBook then read the (four) files. However, I also imported them into iBooks, and iBooks did a far better job displaying the contents. ShuBook did not get the links and navigation right; iBooks did. (I made the comparison to ensure that there was nothing wrong with the original files.)
For me, ShuBook is not a Stanza replacement at all, but a waste of $1.99 for the add-on. Ill probably try it on a few more things to see if it does anything well, but I fully expect to be deleting it in the near future.
heyjohn about ShuBook 1F