Mozilla Thunderbird 2 Alpha 1 Rocks
I upgraded to Mozilla Thunderbird 2 Alpha 1 the moment it came out and I have been very happy with it so far.
One of the main reasons I upgraded was because I was getting annoyed by the broken Search feature in 1.5 (search by "Subject or Sender" doesn't work correctly.) This bug used to mean that I'd have to either flip back and forth between searching by Subject and searching by Sender or use Google Desktop even if I had the folder I wanted to search in open in Thunderbird. Needless to say, it was quite a pain. Luckily, this has been fixed in the latest release and there are quite a few other improvements too.
I like the new notification alerts and renaming "Labels" to "Tags" just makes it all that much more Web 2.0! :P
On a more serious note, the latest release appears to be much more stable and responsive than before. I haven't had a single crash yet. It is, of course, Alpha software, so use at your own risk!
For a full list of changes in Thunderbird 2 Alpha 1, see the release notes.