I recently replaced Evolution with Thunderbird as my main mail client, and I'm quite happy. I admire Evolution's design but it reminded me too much of Outlook. I use Outlook at work. I hate Outlook, with a passion. Most recently discovered Outlook gem: when you start Outlook, if you have any Inbox rules enabled, it begins applying them immediately, without so much as a dialog box to indicate this. It locks up while doing so and will not relinquish control until it's done, no matter what. If one of the rules doesn't work for some reason (say you accidentally deleted the mailbox it delivers to), it continues processing the rule on every message and only at the end informs you with an extremely unhelpful error message. I hate Outlook.

In addition to Thunderbird, I also use IMP and pine, depending on whether I'm at my own desktop, a web browser, or a terminal. IMAP is a beautiful thing (as pointed out above by DrHyde. I use fetchmail for retrieval, good old-fashioned procmail for filtering, and dovecot for IMAP.