I haven't had experience with any mobile readers as of yet, however I do have the same questions that you do. I am going to grad school soon, and one of the reasons I am hesitant to use mobile readers only for textbooks is because I make notes and highlight and such. Hopefully there will be others who have the same thought and experience with other devices!
The cost is a bit high on the iRex models, but I think that it could be worth it anyway. For me I don't see the battery life posing that much of a problem for what I would want to use it for though.