Ok, let's bring some reality into what is turning into a Thunderbird love-in :-)
I've used Thunderbird since v0.4. It has bugs in. Look at the version number! :-) Stability-wise I have had no problems with it, maybe a few crashes in 6 months I would guess, and only when doing obscure things, not day-to-day stuff. My biggest peeve with it is its text wrapping issues with plaintext emails. You need to go into the edit menu and select 'rewrap' when replying to a message which was sent in an unusual format, like say quoted-printable.
If you send in HTML format, AFAIK there are no such issues. I personally hate HTML email, so I put up with the plain text wrapping issues I occasionally see.
This thread on Mozillazine (where you can get tech help with Mozilla/derivatives) may be an interesting read if you are considering Thunderbird.
