The interface is spartan but both intuitive and effective. You can create as many folders and subfolders as you want. The screen displays up to 8 folders/files at a time (navigation inside a folder works by pressing forward/back buttons). Files and folders can be sorted according to several criteria (creation date, alphabetical, etc.).
I've read some complaints about that an extreme large number of files and folders might slow down navigation but never experienced it since I "only" carry around 200 books in the SD card.
As for ApolloXXI, I got my V3R2 from them: nice, fast and friendly service.