I upgraded a couple of days ago and can overall accept the changes and I know it will continue to improve. The one issue that grates on me is I'll look at dozens of emails, one after another and delete most.
In the old version of TB I would click keep the pointer on the Delete and when clicked the focus would go to the next message. I could either delete or tap the spacebar to scroll through the message. This has changed, contrary to how past keyboard shortcuts worked. Now focus lands on Reply and I have to click in the body of the message before I can touch the spacebar.
"Scroll Down Current Message, then Go to Next Unread Message" is the description on the keyboard shortcut page, and I don't think that's working properly, and certainly not how it used to work.