Mozilla Thunderbird is a redesign of the integrated Mozilla App-Suite mail component. Below is a breif list of some of the changesin 0.2:

-Imporved Footprint and Performance
-Improved Default Theme
-Improved Spell Checker
-Redesigned Options Dialog
-Improved customization
-Improved Stability

The release notes can be found here:
The Project's Website:

At this time Thunderbird is still concidered pre-Alpha and should be treated as such. But I have been using Thunderbird as my default e-mail client on both Windows and Linux for a while without any problems.

Linux with GTK2 and XFT:
Mac OS X:

the 0.2 Source Tarball is also available here