I don't know much about the technical details of JFFS2, only that it takes forever to mount after an unclean shutdown and runs painfully, painfully slow on larger flash volumes (i.e. >512MB) as they approach capacity. I wrote a tutorial (mostly so I could remember how to do it myself) for converting a SheevaPlug from JFFS2 to UBIFS because of the performance difference: http://nitrogen.posterous.com/converting-a-sheevaplug-from-j...