(Although I usually kill the default tags that Thunderbird still includes as a hangover from the original Thunderbird release because that way I get useful tag names in the Maildir should I ever need to hack on them directly.)
Oh man. As a longtime Thunderbird user, I have to say, that's a loaded question. :)
I use Thunderbird with a cheap fastmail.fm account. You can access your fastmail account from anywhere with their web interface. You can access it from any imap client.
It all works good enough for me. Not fantastic, but good enough.
Sadly, a full on "here's a hierarchy of tags, just display mail by tag" mail reader doesn't exist to my knowledge outside of GMail itself :(