I use OfflineIMAP to back up Gmail and Google Apps mail. I have it set up to run every evening from cron, backing up a Gmail account as well as a couple Apps accounts. It uses Maildir format.
Yeah, I'm currently using OfflineIMAP until I find something better. But it's really not a perfect solution: it downloads emails multiple times (once per label), it doesn't restore to Gmail terribly well, it's pretty slow, and it crashes sometimes. If Gmvault adds some sort of export facility, I'd switch.
As said above OfflineIMAP will create a blend copy of your emails and you will loose quite a lot of very interesting features when copying them to a standard IMAP server.
That said I understand the need and will think on an approach to allow users to leave Gmail for another IMAP server in v2.0
Another point is that Gmvault is meant to be easy to use. OfflineIMAP is a very good tool but it is meant for advanced users like us as Gmvault is meant for users with very little computer knowledge.
Gmvault v2.0 will go further as it will have a GUI (while still having a CLI mode) to allow my Granma to backup her emails :-).