I've found the nightly builds to be stable since near the end of October and a huge step forward from fennec 1.1

A tip: disable all of your addons in fennec 1.1 before installing the nightly build and then you can safely use the button to force addons to be loaded and enable them 1 by 1 without risk of (e.g AutoPager) one of them crashing the beta. If one does crash the beta you can use the command line to run fennec --safe-mode (from memory, may be wrong!) to allow you to fix it.