Excellent! Rated 4 out of 5 stars
I really, really, like this add-on and have been waiting for something like it for quite a while. Ever since IBM showed a proof-of-concept for this I have been drooling at the prospect of my email client incorporating it.
I think ThreadVis has pushed me away from Mail.app and toward Thunderbird permanently.
The only downside is that it does not yet work with Thunderbird 3.x.
** UPDATE **
I have been talking with the developer and he knows about the problem with "virtual" (saved search) folders and it should be fixed in the next release. Also, it should be 3.x compatible. That will correct both of my major problems with ThreadVis and will probably move me to start using the TB 3.x beta much more.