Server outages like this 1st of September are rare but always bring back some fears like "what if it doesn't come back online ?", "did I loose everything this time ?", "has a hacker managed to crack my inbox ?", and while "real men don't do backup", you kind of regret that you didn't.
There are other good reasons to consider an offline backup: in case you reach the generous online storage limit, but also to store your emails with private information (memberships, passwords, ...) away from a potentially hackable place.
While the Gmail offline feature can store your content locally, it is only ment to synchronize emails (that are less than 5 years old) so you can work offline until a connection is available. If you want to delete the older emails online, they get deleted offline too upon synchronization. So you have to look elsewhere for a backup solution.
A simple and yet powerful approach exists: MailStore Home edition.
This freeware will archive emails from about any source (Outlook, Thunderbird, ...), including POP3 and IMAP services and let you store the backup on CD/DVD or USB storage afterwards. The archive can be searched and emails viewed directly from the application (which is faster than using the original client).
Adding to the product power, and to your freedom, MailStore can also re-export your backup to any of the numerous supported formats! So if Gmail had to disappear or become a payware one day (remember Yahoo POP3?) you could still re-inject your part of all your data into Outlook, Exchange, Thunderbird, SeaMonkey, or another POP3/IMAP server.
The new MailStore Home v.4, features a portable version and improvements detailed here.