First, I want to applaud the Rockbox developers for all the work they have put into this project.
Other than running out of memory when trying to play certain MIDI files, I have only encountered one issue:
I attempted to use the PictureFlow plugin on my Sansa Clip Zip, but no album artwork is displayed, only gray boxes with white question marks. However, my album art displays correctly on the WPS.
I did not see anything in the Plugin Index entry for PictureFlow, the forum, or the Rockbox manual that indicated that anything special needed to be done with the artwork for it to display.
Is there something I missed?
EDIT: I forgot one detail: I am running Rockbox version e61f42a-120211.