I've been using mh-e for... a really long time, and it's completely indispensable for my day-to-day work. MH (nmh) is still the best e-mail client I've ever used (and I've tried a lot of them), and mh-e is the best interface (because, of course, Emacs for the win).

Since I run this project, you'd be concerned if I gave it a thumbs-down review! :-)