Having also been evaluating new mailers over the last week or two, I've a few comments to add to jkeating's mailer comparison:
Evolution supports IMAP over SSH, unlike Thunderbird and Kmail -- it can get at its mail server by running 'ssh $MAILSERVER exec imapd'. That's quite important for me, since the mail 'servers' I use don't actually have an IMAP dæmon listening, and the only way in is SSH. And I don't want my MUA knowing my passwords (or asking me for them either). Authentication is what I keep ssh-agent for.
Evolution certainly used to have an option for not displaying HTML mail. If it's not there in 2.6 then it's a regression. That's not a feature I'd want to live without.
The other thing that I found really suboptimal about kmail when I tried it was the way it handled deletion of messages. When you mark a message for deletion in any other MUA, it stays in the mailbox index. You can read your way down the mails in the folder index, deleting them as you go, and occasionally maybe glancing at the actual text of the mail. But kmail does something really weird -- it first crosses through the message in the inbox as you'd expect, but then it actually removes it. So as your eye is scanning down the folder index, kmail is moving it all around and making it hard to follow. Since I spend quite a lot of time just glancing at mail and deleting it, that's really quite an important misfeature for me.
Evolution can only check mail in all folders, or only the INBOX. That's fairly crap when 'all folders' contains many years of archived mail which is never going to change. What I want is to check mail only in active folders. I have patches to Evolution to do that, and kmail is capable of it too. It can't use the subscribed status of the folders, but you can configure each folder to either be included in the mail checks or not -- and it's not difficult to do that en masse by editing kmailrc directly.
Another thing I want from a mailer is the ability to send patches inline without it eating them. I know that Evolution can do it if you load them from a text file with the 'Preformat' style. I assume that Kmail can, but I'm told that Thunderbird will eat them.
Also, I want my MUA to be able to handle the standard way of sending stuff like party invites to multiple people:
To: Some people : ;
Bcc: firstname.lastname@example.org, email@example.com
Evolution silently drops the To: header, while Kmail tries to qualify it by adding @infradead.org to it, and then complains that ':' isn't a valid character in it. Stupidly.
I also found sylpheed to be a non-starter, because it would segfault as soon as it connected to the mail server.