I have been on the Alpha channel for updates for a long time, but an update yesterday to 2.656 (didnt see the release note btw) has brought me the new UI, which seems to be a real work in progress ie it does not appear to be ready for prime time!
Cannot click on actions to amend them, the menu bar appears but does not respond, etc etc
How can I revert to an earlier Alpha?
Phil
The new UI is not yet functional, however it only opens when pressing option while opening the preferences.
You should however update to 2.660, 2.656 was only a alpha version a while ago. Not sure why you got that, it only had very few users.
//edit: ah sorry, it was the latest version on the alpha channel, but on the stable channel you can get 2.660
Seems to be with the way BTT is launched - if I launch prefs from the menu bar icon I get the traditional prefs UI.
Ironically I had configures a key combo with BTT to launch itself, ie the prefs, and THAT launches the new UI.
Only since yesterdays odd update. It is adamant that 2.656 (966) is the latest available. When it offerered to upgrade, I did notice that there was not a release note for the version is said it was upgrading to.
How can I get 2.660?
Don't worry - found the alpha checkbox in prefs - 'downdating' to stable now!
It's still updating 2.656 => 2.660
2.660 only contains a few little adjustments which is why I didn't push it to all alpha users again.
You probably ran into a little boundary condition where your shortcut to open the BTT preferences contained the option key - which is my internal testing key for opening the preferences 
In 2.660 I have locked the new prefs behind an extra setting.