We did it 2 years ago but I didn't transfer any mail. I just created new accounts for everyone in the gmail domain, did all of our training and then just updated our domain MX records to google so mail started flowing to Gmail.
GoDaddy was only giving our users 100mb of online storage so everyone had their mail locally in either outlook PST or Entourage MBOX. We gave them instructions on connecting from either client via IMAP if they wanted to import their 'local' mail up to their new Gmail box, but very few actually did that. Most enjoyed the chance to start clean while still having access to their old mail locally if they needed it.