Upgraded to the v4.2007 OS yesterday, and had one problem straight off: running plankton meant an entirely broken UI (and invisible keys on the VKB) - had to reinstall Plankton and hildon-theme-cacher, which would've been a bigger issue if I didn't have the Bluetooth keyboard.
Also tried using the built-in backup/restore (for settings only) for the first time. It saved my network connections (good), but not paired Bluetooth devices (bad).
However, worst of all, this morning it's gone into a reboot loop; it gets 3/5 of the way through the boot and then restarts. It sounds similar to that reported before when the RSS reader applet was crashing the desktop at startup. I've not changed any of the feeds, so it'll be interesting to see if anyone else is experiencing this.
Update: it turns out it was my own playing with my configurable font size control panel applet that was causing the problem:
~/.osso/current-gtk-theme may look like a gtkrc file, but it gets parsed to strip out the current theme name to pass to matchbox when starting it. How disgustingly hacky.