After using the new firmware for a while, I can confirm that it's clearly not better than the other ones on the major issues. Occasionally the M90 crashes, exits the current document or just randomly opens the SD card folder. Furthermore, I've had two cases of
deep discharge: just left the M90 in standby and forgot to switch it off. After one or two days in standby, the device wouldn't wake up since it had drained the battery. The only remedy was to fully charge it, which takes around 8-10 hours and a reset.
I think a worthwile accessory would be a professionally bent paper clip that fits snugly into the Boox' pen, for resetting purposes
I'm not sure the above bugs were there in previous firmwares, but they're there in the latest, dated 2011-09-02. Sad but true.