I've got a new Sony SF-32N4 / SF-32C4 32GB Class 4 SDHC card.
First I did some tests on my Linux PC writing and reading back some 2GB of "typical" epub, pdf and mp3 files (organized into subdirectories, "typical" file size was several hundred kB to several MB per file).
The "averaged" writing and the reading speeds were both at about 4.5 MB/s.
The reading speed is a bit disappointing. Sony explicitly claims for this product ... "Up to 15 MB/s transfer speed; write speed lower." ... well, I've forgotten that "make.believe" is one of the trademarks of Sony Corporation while "seeing is believing" is not.
Then I inserted this card (with about 2GB of "typical" epub, pdf and mp3 files) into my M92.
Up to now (just some hours of usage) I see no problems when reading books.
There is one thing that seems strange to me.
After each power-on, the first time I try to enter the "SD Card" item on the "home screen", it takes about 7 (seven) seconds before I get the card's contents displayed (the "main" / "root" directory of this card contains just around 10 subdirectories, nothing else). Subsequent actions are "fast", however (i.e. when I "exit" the "SD Card" item / "return" to the "home screen", and then I "enter" it again, I can see the card's contents displayed without "delay"). Also traversing the subdirectory structure seems always "fast" (even on the first try).
On the other hand, a similar "problem" exists for the "Internal Storage" item. After each power-on, the first time I try to enter it, it takes about 3 to 4 seconds before I get its contents displayed (just some 5 subdirectories, nothing else).