Hi.
I'm a K-Pro owner.
Fragility: it is a very solidly built reader. It is built like a high-end tablet (including metal case) rather than like most e-Readers. Far better than my previous eReaders (PocketBook 360, then Onyx-Boox i62HD).
I'm non-US, and don't use Calibre, so can't comment on those issues.
On the topic of Android version, if you want an eReader with newer-than-4.0, you may wish to look at the Russian variant of the Kepler Pro, the Monte Cristo, which comes with 4.2, and which they also sell through their US subsidiary (
MC3 listed on their website, but only
MC2 currently listed for sale).
The only major issue I have with the Kepler Pro is its button placement. The buttons are placed on the sides of the eReader closer to the top than to the bottom, which I get around by reading with my eReader permanently inverted (which Alreader allows), and the sides of the eReader curve back, meaning that the buttons are slightly recessed behind the front of the reader, making it slightly tricky to get a registered 'press' of the button without it registering as a 'long press'. (The MC comes with completely different buttons, which I have not tried, nor can I even remember reading a review of.)