I have been using 10166 on my 640 as my daily driver for over a week now, and it has been pretty stable for me.
1) The latest core app updates have helped a ton.
2) I still get freezes after some calls.
3) I don't like how close the Home soft button is to the space bar in SMS. Am I missing a setting somewhere to auto hide the soft buttons?
4) There seem to be some notification issues when the phone is dark.
5) Battery life in very acceptable and the expanded notification center is awesome.
Other than that, the biggest issue for me is the functionality differences in the apps I tend to use the most - FlyDelta, Concur, and TripIt. For me, it isn't a deal breaker, but I can do much more in those apps on my Droid tablet than I can with the Windows Mobile apps. I can see it being an issue for some.