Huge problem: after reading a few pages, I usually get sent back to the bookshelf. I tried many firmwares. Many users reported that problem, but the manufacturer and resellers do not care!
Some users suggest buying a genuine Nokia battery.
No problem with the OpenInkpot firmware, but it is very limited: no .doc, no mp3, basic PDF,...
*** EDIT ***
Problem solved with this new great firmware from Ukrania:
http://lbook.com.ua/downloads/V3upda...20090409-2.rar
*** 2nd EDIT ***
Well, I thought my problem was solved but I was just reading a book on the SD-card and suddenly sent back the the bookshelf of the built-in storage!
Which never happens with OpenInkpot.
***3rd EDIT ***
It seems that when the battery is full there are less crashes. And this new LBook firmware must draw less current, but not as little as OpenInkpot. So I definitely must buy a high-quality original Nokia battery...or wait for the next LBook firmware.