I use Courier. It can certainly handle load and it's not a PITA to configure.
I used qmail for a while many years back. It was an interesting learning exercise, especially adding "extra" things so I could do things like local relaying. However, I rebuilt my server some while afterwards and looking at all the effort required to get qmail going again, I asked "why am I bothering?" By then, I had found Courier-IMAP to provide IMAP access to my maildir storage and in the interim the rest of the Courier MTA had been released. It was much easier to install and much much easier to configure than qmail, so my decision was made.
I haven't tried Exim, but I had to work with Postfix at one job and was very annoyed at its complex configuration. I had to make it accept email for non-existent users and pass it to a script for our application (which handled email intermally). Postfix made this unnecessarily difficult IMO, but the sysadmin wouldn't countenance a change to another MTA.
The only wrinkle with Courier is that the cutting edge MTA tools (e.g. SPF support, grey-listing) are a bit behind everything else because of its small share. And it doesn't get on 100% with fetchmail sending to it, for some reason I couldn't figure out.