The printable area on a DIN A4 is typically ca. 170mm x 260mm while the M92 has 148mm x 210mm. While most books (including non-fiction) are not DIN A4 or the margin is at least significantly larger, the M92 is sufficient for this purpose (print magazines are often problematic). Sheet music often comes in "real" DIN A4 format or even bigger and with a pretty small margin: e.g. the German publisher Henle has a printed area of around 195mm x 260mm - that's much too big for the M92.
http://www.abload.de/image.php?img=201203110398rk7g.jpg
http://www.abload.de/image.php?img=20120324041z4kdo.jpg
But for the rest, the M92 is perfectly okay: Once you get used to pressing a button instead of really turning a page, this gesture is okay and the delay is not too long. Besides that the M92 doesn't have any problems displaying (poorly) scanned pdfs or pdfs that are directly exported from software like MuseScore. (I'm using sheet music from imslp.org and never had problems with my M92.)
I only use the M92 to look up sheet music that I can play already. For learning a new piece I always use printed sheet music, because it's significantly larger and feels better for my eyes. After having learned a new piece I put the printed sheet music in an archive. Afterwards it's usually faster and easier to access the sheet music on my M92 whenever I need to glance into the notes because I forgot how to play a certain passage.