Wow, look at me with my first non-Drupal related post. ;)
I recently upgraded to Leopard and since then, Mail.app hasn't worked properly with my crappy e-mail provider, so I decided to give Thunderbird another shot. I like a lot of features in Thunderbird, including the fact that it's open source, it has useful extensions like Enigmail, and it doesn't take forever and a week to retrieve my list of new messages like Mail.app does (or did under Tiger, anyway).
Of course, the one major problem Thunderbird has is that it sometimes takes forever to load a message body. Or at least it did, until I literally Googled for thunderbird takes forever to load a message body, and came across this gem in the comments of a lifehacker story:
Click on Tools then Account Settings. Look under your IMAP account for "Offline & Disk Space".
Check the box next to "Make the messages in my Inbox available when I am working offline."
This will download the messages to your hard drive (just as it would with POP3). This makes it load instantly in your email client, while maintaining the advantages of IMAP.
You can also select other folders to be available offline by right clicking on the folder and select Properties. Then select the Offline tab.
You get the speed of POP, multiple platform sync, your messages stay on the server, and are backed up on your hard drive.
Thanks, rHughes :)