@kleykenb: Note that the firmware of the M90 is pretty much the same as the one on their current models, so if you want to know more about how it handles PDFs or other formats, just ask around or look up the 6" models. From the videos that I've seen, it seems to be one of the best PDF readers around.
Regarding the battery life, I too am concerned that the M90 will have poor battery life. I think that it uses the same CPU that the 6" models use and their battery life is pretty crappy compared to the Kindle.
The M91S on the other hand, supposedly has 45 days of standby time, so that might amount to 2 weeks or so of use? (kinda like the Kindle DX I guess).
Regarding the internal memory, it doesn't matter, you are going to use an SD card to expand it anyway, so who cares if it's 1GB or 2GB.