I've run my own mail server for 15 years, since I got my first permanent connection. I host on the end of it as I have a large distrust of "the cloud".
It is cost effective for me as it has increased my merchantable skill portfolio. I've ended up designing some mail systems (50k+ users) for some large ISPs in the past thanks to my accumulated knowledge.
Debian is probably the easiest to get off the ground - it's pretty much "sudo aptitude install postfix dovecot" and follow the instructions. I was a FreeBSD user but primarily due to apathy, I tend to use Debian.
This is about to change however, when FreeBSD supports the raspberry pi as it's a much lower memory and power footprint device so some of FreeBSD's simplifications and optimisations will assist there.
For me, a Raspberry Pi with a 32 gig SD card plugged into my 12Mbit connection will suffice for the 18 users and 5 domains via IMAP that are currently being hosted on a much larger machine. Cost to me: $40-50. No brainer.