I've been using google hosted for my personal email for some time now. Cheryl was using their web client and I was fetching all my mail over pop to a local dovecot server.
After I heard they were going to support IMAP I decided that maybe I will finally migrate all my emails (back to 1996) to the google servers.
I noticed that messages copied via imap had incorrect dates when viewed from the web client. That hindered my decision for some time, but Andrew mentioned that they were going to eventually fix that. The dates still appear correctly in imap cilents, so I wasn't too worried. I'll mostly use an imap client, but it will be nice to be able to check and send mail from a web client.
(When hosting my own mail with dovecot, I had squirrelmail set up, but my mail was often rejected because it was sent from a dynamic ip. The unreleased squirrelmail beta had the option of configuring one authenticated account for outbound smtp, but using that feature with gmail was a little clunky because it seemed the mails weren't masqueraded properly.)
One of the things I really like about using gmail over imap is the ability to tag spam by moving it to the [Gmail]/spam folder. I had pretty good luck with spamassassin and although it was fun getting to work, I had some false positives and decided I didn't really want to think about spam any more.
The last of my concerns were answered by this help thread:
I just hope I don't start deleting messages while using other email servers and expect them to be in my 'All Mail' folder :)
The performance is ok, but not as good as using my own dovecot server serving one account. But since I get the above features and I don't have to worry about backups or my computer going down, that's something I'm willing to live with.
Some people have asked how I did the actual migration. I configured two imap servers in Evolution and manually copied messages/folders from one account to the other. This took several hours of babysitting the process for roughly 250MB of mail.
It may be worth looking into imapsync.
Andrew sent me this: google-email-uploader