AN.ONYMOUS  Has it finally corrected the "Determining which messages to index" that hanged the download for hours and wasted most of the inbox mail and sent mails that were transferred from another machine ?

If it does not, I shall be obliged to give up with this once nice application.  
(Version 3.1b1)

B. JEFFERSON LE BLANC  I don't like the name change. It seems gratuitous. The version number is the standard place to indicate a beta; distributing Thunderbird with a "code name" just confuses the issue. Worse yet, it breaks Penelope yet again.  
(Version 3.1b1)

WAYNE WELSH  Versions 3.0.2 and 3/0.3 have both re-imported all my emails after installing and starting it up. That's very aggravating. I have to let it run in the background for 30 to 60 minutes or so until it's through. I have about 9600 emails at this point. I'm not looking forward to this happening every time I upgrade Thunderbird.  
(Version 3.0.3)

ROBERTCOOGAN2  Huge difference with this latest update to v3.0.2.

Very stable, and of the bugs I previously reported are gone.

...and it's back!

One suggestion, there needs to be closer integration with Apple Address Book. The ability to add/delete/change info in the Lightning extension and Google contacts from within Thunderbird/Lightning would be a huge help.  
(Version 3.0.2)

SMTIPS  It seems faster, but it's REALLY lame that they added those buttons on the right, above the message pane, using up 4 or 5 lines that simply don't need to be used in the header area. AND they give you no avenue to turn them off...DOUBLY LAME.

Thil the dumass devs actually add a way to take it off (WHY are they so stupid as to add something like that without the ability to hide it it completely beyond me) I went back to 2.0 which has given me zero issues for years.

This kind of crap is ALA-MICROSOFT saddling a user with something "they think is best because we know what the users will want" when in reality they are just trying to force-feed some dev's little pet idea onto the community.  
(Version 3.0.1)

HOLMEMOSS  v3.0 is still telling me there are no updates available!  
(Version 3.0.1)

RAMPANCY  As much as I like the many improvements made in TB3, what annoys me to no end is the rather dismissive attitude that the Thunderbird developers have had for SpamSieve's developer Michael Tsai when he commented on the changes made in 3.0 which broke his plug-in. They just don't seem to care about helping him, which is a shame since SpamSieve's filter is far superior to TB's own spam filter.   
(Version 3.0.1)

BOUSOZOKU  For the most part, the upgrade to version 3.0 was a good one.

Searching has changed dramatically and confused me at first. By default, Thunderbird 3 searches everywhere rather than searching by sender or subject as previous versions did. Clicking within the search field allows you to select the earlier default.

The other dramatic change is the combined inbox. There was a rather pesky problem with this. I would delete an e-mail and it would not skip to the next e-mail in the list. That's been corrected as of version 3.0.1.

Something that has stopped people from upgrading to version 3 was the lack of Lightning compatibility. That was rectified recently by version 1.0b1 of the Lightning extension.

As always, there will be room for enhancement and version 3.0.2 has been in nightly builds for a week or so. I believe that most people who need a capable e-mail client will find it very usable and those upgrading will now be happy with version 3.0.1.  
(Version 3.0.1)

JAMUS  I see quite a few bad experiences with TB below, but oddly enough, I have had very good experiences with it. There are some odd GUI choices I do not prefer, but for the most part it has worked very well.  
(Version 3.0.1)

ROBERTCOOGAN2  Agree with previous poster, this is bug-riddled. I am glad I still have the installer for 2.X.

Advise all to at least wait for next version, this is a very good app that deserved better than this seriously flawed release. Not sure what Mozilla was thinking, I hope that this was a mistake...

Problems:

-Window wants to resize itself every launch. Does not remember previous size setting.

-Action buttons (reply, reply all, forward, etc) over each e-mail are repeated three times. Last row is dimmed and buttons do not work (not that they should!).

-Certain From e-mail addresses, subject lines, and sent dates are blank in main window at random. Some are there, some are not. Changes with each launch.

-Attachments do not appear at all.

-Will not quit. Have to Force Quit to stop the app.

I stopped there, I recommend that all wait until bug fixes are out. Stick with 2.X for now.  
(Version 3.0)