15.1 crashed all the time on my PB 360, but almost never on my wife's. We use it about the same amount, and both used an enhanced FBReader. The only difference was I put my books on a microSD card & hers are in the main memory. So, this is a good example of YMMV.
I downgraded both back to 14.2 awaiting the next stable firmware update. Interestingly, mine still locks up every now and then and hers never does. Given this, I'm suspecting that the microSD has something to do with the stability issue in both firmwares, but that 15.1 is especially prone to the problem.