Hi,
there is existing support thread for PRS+
here. (name is misleading a bit, I admit).
There were bugs related to opening fb2, but all known ones where fixed.
Built-in converter fails to convert about 10% of the fb2 books though, but what we've seen so far all of them were mis-formed (usually links are broken). Changing converter to be less picky about following fb2 standard is planned.
I hope this helps.