I don't have a BlackBerry, but it is in the same Forum at MobiPocket as other Java-based Readers which I am tracking. The standard version gets more questions in the forum than EInk devices, but there are way more BlackBerries out there.
The beta version has been designed for the Storm, and, for a beta version, I don't see a lot of issues on the forum. See
Mobipocket Reader for BlackBerry - Beta.