I've just done a quick convert of an epub in Calibre to fb2 and installed in on my X6 and also put a text file in it.
Text file looks ok. As long as you lay out the text so that you can tell where the paragraths are then it displays ok.
FB2 loads but for some reason my title page failed to display (it flashed up for about 5 seconds and then disappeared). Apart from this though the contents displayed correctly with bold and italic text show correctly and looks pretty good. Very passable (for myself) as an emergency ereader, as your main ereader, it could work, certainly if like me you like carrying one with you at all times. My PRS-600 is too big to stick in my pocket, but this isn't.
As long as you've got somthing like Calibre to convert your books, this could definetly work.