There are a number of known bugs. This page mentions that the majority will be fixed in a week or two: known issues in After Effects CC 2015 (13.5) | After Effects region of interest
This page discusses the change in previews: all about previews in After Effects CC 2015 (13.5) | After Effects region of interest
If it doesn't work for you, simply use CC 2014 until the update for CC 2015 comes out. In case whoever updated your machine told it to remove old versions and you don't have CC 2014 anymore, it's very easy to install it again, see this page.
I had read all that great info, but was hoping a downgrade wouldn't be the solution.
Oh well. That's what you get for being an early adopter i guess. I'll stop being so keen to install upgrades. :-)
Your description of 'absolutely unusable' to me is pretty accurate. I've never had so much trouble with an AE release than I have with this one.