On the plus side, this works. I have it set up to archive my emails several times a day, which it does quietly and unobtrusively in the background, without hogging the processor.
It's really reassuring to know that every once in a while I can delete six months worth of email out of Mail to clear out the cobwebs and keep it ticking over smoothly, but the archiving is already done. So all of my invoices, software serials, business communications, and so on, are still there for me to access, without me having to take anything more than set up MailSteward once, then let it do its thing. The first run takes a while, and after that it goes much more quickly.
It really has been, for me, set and forget.
The main negative for me is speed of searching. Searching the archive for the first time after MailSteward boots up is slow. I'm talking minutes, not seconds. But everything's relative, and I have quite a lot of email in that database by now. I hope the dev can find some way to make that work better.
On the whole, however, this app has proven the best for me, and I've tried a few.
(MacBook Pro 2.53, 4GB RAM, OS X 10.7.1)