Works excellently... Rated 5 out of 5 stars
I use Enigmail 0.94.2 with Thunderbird 18.104.22.168 (the versions supported by Ubuntu linux) and it works excellently.
The integration is seamless, even having little details like the "this message will be signed" and "this message will be encrypted" indicators in the bottom-right of a new message, just like how Thunderbird works with S/MIME.
Having one-click decryption and verification is also nice. The Enigmail key manager is also excellent, though I find it lacks some features (like being able to select a default key) that must be selected with Seahorse. Also, having a user-configurable time for caching passphrases would be nice; 5 minutes of non-idle time is too short for my needs.
Enigmail's easy-to-use frontend to GPG and its excellent integration with Thunderbird make this add-on a most excellent choice.