For over twenty years, Program for Internet News and E-mail (Pine) has been my favorite e-mail client. It has performed exceedingly well and, in my [not-so] humble opinion, is all that an e-mail client should be. In other words, I really have no use for a GUI-based application to handle my e-mail. I’m also a pragmatist and, as my requirements for an e-mail client have evolved, Pine, unfortunately, hasn’t — at least not quite as much as I need.
I recently used Apple’s Mail.app to compose an e-mail, which is difficult for me with respect to using the keyboard outside of the superlative editor, Vim. If that were the only issue, I could probably adjust to using a so-called normal editor (like those that exist in every mail client). The problem with making the switch from Pine, specifically, to anything else is that Pine has some features that make it a superlative mail client. The two most important features are the ability to store configuration data and my address book data on the mail server. I think I would actually pay for a non-buggy (e.g. one that can accurately both sign and encrypt a message, or do either with an attachment) mail client that would implement those features. If it allowed me to, additionally, use Vim keystrokes, I would pay even more for it. Fundamentally, it doesn’t make any sense to have to configure mail server settings on each and every computer that one uses.
Update: This entry has been sitting in the draft queue since mid-2013. Since then and due to the aforementioned limitations/bugs in Pine, I have been using Mail.app (both personally and professionally) on a more regular basis, but I still use Pine more often because of how much more efficiently I compose e-mail with it since I can use Vim as the primary editor.