The background code changes are really really nice.
The UI... I dont know. The sidebar is still useless and not a replacement for Tabs. Everything got even bigger, and TB Coversations is still better than their Threads implementation I think.
But I have to look at it again.
Would be cool to have a stub mail account to test it with, but I just use my main one with IMAP.
I stopped caring about Thunderbird a long time ago, but when Thunderbird Sync was announced, I began to reconsider. I loved what I saw of the desktop redesign, but was super excited for Thunderbird for Android. I'm happy for the progress they're making, especially since said changes are pleasing for your user experience.