I use an ASUS ME572 that I bought from Best Buy. So far it's been stable.
I honestly haven't tried Calibre Companion or Pocketbook. I mainly bought it to use as a reader.
It periodically runs on sale for $149.
Code:
http://www.bestbuy.com/site/asus-me572-intel-moorefield-16gb-black/8769033.p?id=1219365672438&skuId=8769033
I've also found a lot of the Nexus 7 FHD (2013) cases will fit it. You just have to make certain the corners aren't covered if the case has a slip in area for the tablet. The ASUS tablet has squared corners.
I found this case on ebay and it's a perfect fit:
Code:
http://www.ebay.com/itm/181411627876?_trksid=p2060353.m1438.l2649&var=480389441733&ssPageName=STRK%3AMEBIDX%3AIT