Thunderbird finally drove me over the edge. Might have been that whole "no new features OH HEY YOU CAN IRC FROM YOUR MUA" - guys, if I wanted emacs, I'd use it.
I used to care a lot about Enigmail. For various reasons, I care less about it now. There are, however, a few things that I would miss about it. Oddly, the one that I kind of missed the most is the most whimsical - I like having my default signatures rotate. So I did a bit of digging, and came up with some applescript (courtesy clapper.org's post on the subject) only slightly modified.
Also, being a complete Mac-centric scripting n00b, I wasn't sure how to make things go. Save the Applescript into a file called… anything. Run osacompile against it. You can call the resulting compiled script from your .bashrc with osascript, something like
osascript /Users/foo/bin/sigrot.scpt
Another relatively minor irritation is the default behaviour of never marking mail as read, or marking it as read instantly. You can theoretically fix that with
defaults write com.apple.Mail MarkAsReadDelay 4
but that didn't work for me (10.8.1). Instead, I used TruePreview.
Now, if only I could convince it to show me messages most recent at the top, but when they're threaded, show them most recent at the bottom.