I actually just switched from Apple Mail back to Mutt, because a combination of Mail and an Exchange server ate a huge chunk of my email archive. I'm not sure if I'm really more productive, but I feel a lot more comfortable knowing I have a degree of control of what's going on, and that stuff is being stored in an open format (Mail switched from Maildir to something weirder a while back). Losing a chunk of my email archive was pretty traumatic.
The main thing holding me back was a decent email search feature -- I'd been watching notmuch for a while, and when I heard about muttkz, I compiled it and switched. I use davmail, offlineimap and muttkz. I use notmuch to search around 10 years of email.
I don't think this is a route I'd recommend for many others -- I've used mutt for years before Mail, and only switched over in the last couple of years. But it worked for me, and you did ask.