I moved back to it from TheBat! a few weeks ago and must say I'm impressed. Or at least very satisfied. I forget why I moved away previously but so far Thunderbird is doing everything I need it to. Its IMAP support is a lot better than TheBat!'s in that it actually works and works well. No random connection drops, no random inability to download message content and TB is happy to work with large folders like Trash - I was beginning to think a couple of my archive folders had got corrupt but no, a decent email client shows the contents to still be there and fully accessible. The Thunderbird UI is a little odd but it didn't take long to get to grips with it and it's snappy enough.
Another example of open source software being better than a commercial rival. I'm off to make a donation :)