There are definitely bugs in places, but overall it is pretty great so far. The task switcher seems particularly buggy to me (screen's frozen a couple of times, but just needed to turn the screen off and on to fix).
What I wasn't expecting was how many changes they made to core apps. Lots of UX changes for better or worse depending on your perspective, but I welcome the smaller text and items and overall clean layout. Added functionality in apps like mail, phone, and messenger are also welcome and core to your daily usage. Spartan is cool too, it renders better than IE and seems to be quicker/smoother.
One observation though: I really pay much mind when people were making a big deal about moving menus and stuff to the top of the screen rather than the bottom, but I can now say that it does detract from the user experience a little. I can't comfortably reach important menu items at the top when using this phone with one hand with my normal grip.
Overall though I am pleased and didn't care much about installing it on this phone anyway as I'm almost due for an upgrade and can switch if I brick this.