I’ve used Thunderbird for a couple of years and recently moved to TB3. It’s not been without pain though, and here a couple of the issues and my workarounds
Indexing – lots of talk about this on the web. Despite leaving it going for over 24 hours, Thunderbird 3 had not finished indexing all my folders. I discovered the location of the index file. Mine is currently over 2GB in size. Not good for Windows! I’ve therefore disabled indexing. However ideally, I’m happy to have the index in place for my main folders, so I’m using the GlodaQuilla extension to selectively index folders. Update 01/2010. Given up on indexing altogether.
Calendering. No built in support, but nightly builds are available for Lightning (and the Google provider).
Activity Manager. At last I thought, a way of seeing what Thunderbird is doing at any given time. But no, it only reports after the activity is finished.
Update 01/2010. Chief gripe now is that search box doesn’t have any kind of pause/delay built in, so as soon as you type the first letter of your search term, Thunderbird grinds to a halt while it list every single email with that letter in it. Having to cut/paste search terms as a workaround.