Not perfect. A clear improvement, however. I particularly dig the new search. Once I figured out I could configure the Archive function to put archives from all my accounts into my local Linux box’s IMAP server I also liked the new Archive function. It nearly matches the system I maintained by hand anyway (manually periodically moving a bunch of mail into suitably named folders on my local Linux box).
Also nice is the “smart” Inbox — a feature I’ve liked in Mail.app and kind of missed in Thunderbird.
I’m not yet sure if I like the rest of the UI changes but I’m going to try them out for a bit before trying to reconfigure them back from whence they came.