I use unrealBook as well. Have been since it first came out. The developer is very responsive, and is a musician himself. It is purely a PDF viewer though; I don't think it can "play" a score off a sheet or transpose a sheet; it does not use some 'custom' format - just regular PDF. But very flexible in terms of features for musicians - Set Lists, MIDI support, metadata/keywords (database-like features) for things like genre, year, style, composer/creator, lots of other data fields, etc.). Lets you annotate a page etc. Lots of features for sending files to other musicians, sending/receiving files, sending/receiving Set Lists with others, etc.

If anyone using urB needs help getting started etc. feel free to send me a note and I'll help if I can ...

-Jim