Some of us do read in the dark. Personally, I use it extensively when reading in bed at night while my partner sleeps. Additionally, the coverlight gives off a bright white light which enhances contrast compared to normal "yellow" moodlighting.
I have no experience with the DRM hassle though, since I use mine exclusively with DRM-free books in combination with Calibre. A lot of those can be found on
Project Gutenberg.
As for PDF's, you got me there: those rarely work flawless for me too. Sometimes converting them (again using Calibre) to a more compatible format works, but a technical manual with images etc is no picknick to read on a 6" device anyway - you're better of buying one of the bigger readers for that.
At any rate be sure to inform Sony why you are returning it, eg. the DRM trouble. If enough people do that, who knows, perhaps they will finally give up on that notion some day.