I started out with Exim (3.x or 4 -- can't remember now) and stuck with it mostly for the same reasons that Debian didn't change away -- I wasn't convinced postfix was a significantly cleaner/simpler design -- that it would be worth it to learn how the "other" system worked.
I did have some exposure to qmail before that, and it did feel like it was simpler in some good ways -- but then it became a sort of abandonware, sadly.
[edit: I did recently switch a few vps' over to nullmailer rather than use exim4 configured for smarthost delivery -- once I figured out how to get tls working thanks to this blogpost http://metz.gehn.net/2012/11/nullmailer-with-starttls/)]
I did have some exposure to qmail before that, and it did feel like it was simpler in some good ways -- but then it became a sort of abandonware, sadly.
Any thoughts on Lamson ? (http://lamsonproject.org/)
[edit: I did recently switch a few vps' over to nullmailer rather than use exim4 configured for smarthost delivery -- once I figured out how to get tls working thanks to this blogpost http://metz.gehn.net/2012/11/nullmailer-with-starttls/)]