[antoncohen]

I've done a few migrations to Gmail/Google Apps, I'm actually in the middle of one right now. It's pretty easy to retain your folder structure, especially if you have IMAP and are going to Google Apps. Nested Labels has been moved from Labs to a standard feature. I think there is a still a 40 char folder path limit, so make sure your folder names are short enough before migration, e.g., rename "Mailing Lists" to "Lists".

Google has the Migration for Microsoft Exchange tool, which does IMAP sync with any normal IMAP server. The tool is Windows-only, they used to have a web-based version but they discontinued it (WTF Google?!). It requires 2-legged OAuth, which requires Google Apps for Business. But Google Apps for Business has a free trial with no billing info required. So what I do is sign up for Google Apps Free, upgrade to Business, do the migration, and downgrade to Free.

If your mail in stored in a mail client (POP3-style), you can add Google Apps IMAP to your mail client, and drag and drop your folders/mail from the old account to Google IMAP. Be sure to hold down the Ctrl key (or whatever) to copy not move. This method also works if you are going to normal Gmail.

There is also imapsync, a Perl command line tool, and a few other tools that do things like Maildir to IMAP sync.