Just checked back on the world's best email index/searching system, and discovered that in addition to Maildir and MH folders, Mairix now supports mbox folders.
So all you saps who didn't upgrade to Maildir when I told you to can now find out what all the fuss is about. :-)
Simply put, Mairix has revolutionised my use of email. I have over 250,000 emails in various maildir folders, and Mairix allows me to search and retrieve on all of them within about a second or two. So whereas before I'd have to hunt with a mixture of my own memory and grepmail (which took ages), I can now identify the email I want almost immediately.
Bloody marvellous, that's what it is. Insanely great.
But don't take my word for it -- try it now, and find out for yourself!