Just a quick update.
Google's offline mail, via Google Gears, works well enough for advertised purposes, I guess, but I don't really trust it as a backup mechanism ... several emails on line don't seem to have made it to local storage.
Gmail Backup was working pretty well at first, but the latest version - 0.107 as of this writing - changed several elements of the backup process. Those changes made it pretty well useless to me. Four+ years of email does not store well in a single directory.
Postbox, a commercial product currently in free beta, doesn't properly handle mail with multiple labels/tags. Since I'm using Google's label system to categorize and segregate mail into multiple areas, this product, as well, fails to satisfy. Perhaps it'll improve as the beta moves forward, but currently it is nonfunctional for my purposes.
MailStore Home simply wants too much attention. It insists on running in the foreground, and seems slow, as well. If I switch to any other desktop - or even just a browser to check current mail! - it goes belly up. I don't know whether it will do what I want or not, but I just don't have time to deal with such a prima donna of an app, one that doesn't seem to understand the multitasking concept.
That leaves only Gmail Keeper, a commercial product, yet to try ... there doesn't seem to be anything else around <sigh />.
I'll be back with an update on that as soon as I've got something worth mention.