I use it and I pretty much like it.
I did have a problem where something crashed (can't remember the circumstances) and it appeared to have 'lost' all my e-mail. I was able to recover, but it was a pain. In the course of researching it, I found that the problem was a file, prefs.js, that was corrupted and a common issue.
Since then I made a backup copy of the file. The corruption happened one more time and all I had to do was copy the backup over to the original, restart TB, and I was back running.
Here's a link that describes the problem:
http://kb.mozillazine.org/Recovering...ly_disappeared
So if you do use TB, I recommend backing this file up as soon as you get all your profiles set up the way you'd like.