I like Thunderbird. I used Outlook for a bit, but went back to TB because Outlook seemed much slower and more bloated. (Outlook also has loads more features, but most of them aren't going to be used outside of an office environment.)
Thunderbird is small, easy to use, and I don't think it's ever crashed. What's to dislike?
I get the feeling it doesn't receive much love within Mozilla, though. It must be a much less complex undertaking than Firefox, and yet we haven't seen a new version in over two years?