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)

ECONKLIN@FORTH.COM  I have used Thunderbird on both Mac and PC ever since it came out, but 3.0 is seriously flawed. It is SO not ready for release! I have a list of bugs as long as your arm, and searching online I find other users with the same bugs on various sites, and no relief in site. I want to go back to 2.x! Fortunately, I still had my old 2.x install for Windows, but not a Mac version.

Latest example: it won't open an attachment if it doesn't recognize the application, and there's no way (that I've found) to add an associated attachment.

Also: its main window doesn't resize properly.

Many other issues.

Very disappointed!  
(Version 3.0)

BCWINTERS  I have found Mail.app almost unusable at work due to its handling of attachments, so I thought I'd give Thunderbird a try.

While I like it conceptually and am happy with the majority of its features, it's pretty sluggish compared to Mail. This is despite the fact that my Mail database of personal correspondence includes more than a decade's worth of POP mail, while Thunderbird only has messages from the last two months.

The vaunted search capabilities often leave me hanging--the "summaries" are too short and trying to open them almost always leaves me with a blank window rather than the message I need. The threaded message view is also frequently missing messages.

When composing mail the cursor starts out in the body of the message and then jumps to the To: field, so the first few letters of an address end up in the wrong place. The stylesheets also change on me while typing a message and cursoring around, so half my email ends up in a different font.

Thunderbird is not yet a viable competitor for demanding users but it's getting close. I'm very glad development is continuing and look forward to updates that take care of these and other issues. I consider it the equivalent of the 1.x branches of Firefox--it took until v2 for Firefox to get really good and v3 for it to get great. Hopefully the Thunderbird team is learning from Firefox and will progress speedily.  
(Version 3.0)

ZEEBIE  Looks nice, want to use this as newsreader, but I'm getting mad trying to add custom headers. Why has it to be so complicated?  
(Version 3.0)

BPM  LOve the new look and everything!!!!   
(Version 3.0)