I flashed from 15D to 15G yesterday, and wound up flashing back to D again. I should have evaluated G before posting that it was available, since Ectaco seems to have no quality control. It looks like the back flash was successful, but I have not done a full evaluation yet.
The problem with G, at least on my JBL, is that if you enter the machine from the main menu (Books Pictures Settings screen) and open a book, it has lost the page number where you left off, opening on page one, and it resets the font size to 16 no matter where you had it set. I did not need to look any further to see what else it does.
Nice work Ectaco! After waiting since the last firmware update in January, this is what we get.
Also I ran into this post where a user claims his unit froze on version D.
http://www.ectaco.com/ectaco-posts/list/19895.page
D was supposed to be the version that fixed this problem. I have been using mine with D without issue, but now I don't trust it.