I've decided to change from Thunderbird to Evolution for my emails. I've got frustrated with TB's many lacking features, and consider Evolution to offer a much more complete feature set. For the timebeing it's working nicely, and I've already started synchronising my IMAP folders.
Evolution and Thunderbird are both GTK+-based email clients. I'd love to use Kmail or another Qt-based email client (I run KDE4.1), but can't justify the lack of features for the aesthetic advantages.