I have recently moved to Google Apps.
One of the nice things is that you can have IMAP access so you can keep using the MUA of your choice.
If you have the paid version (Premier Edition), IMAP migration can be done smoothly using the migration tools, which suck messages from your old IMAP account and transfer them into your Google Apps account. This has some interesting benefits, such as de-duplication of messages. I used to copy all incoming mail (after spam etc filtering, just before it hit my INBOX) to an ‘All’ mailbox in my old IMAP account. I could then keep my INBOX clean by just deleting messages, which were still available in the ‘all’ box. This was of course not very efficient, and in the end it contained about 100.000 messages which were well over 2 GB. After migration to Google Apps, all double message were removed, and instead they just received an extra tag ‘All’. Very cool.
If you have the free edition, you will have to do the transfer yourself. There are command line tools such as imapsync, which work most of the times.
You can also configure two IMAP account in your MUA, and then copy folders over. Whatever method you use, there are a few things that can screw up the migration, and that should be fixed prior to any migration attempt. If you don’t, you migration will fail somewhere half way, and your MUA might or might not inform you what actually went wrong.
Messages greater than 20 MB cannot be stored
The easiest way to fix this is to remove those messages from your mailbox and copy the attachment to your computer. Sending such huge message is highly questionable at all, and the good thing is that once you are in Google Apps, you are not allowed to such ridiculous stuff any more.
IMAP folder names cannot exceed 40 characters
This is an implication of the fact that IMAP folders are merely a representation of labels, which cannot exceed 40 characters.
Hence the total IMAP folder name cannot exceed 40 characters. Also, each level counts as a character, because it is represented by a separator (/) in the label.
This can seriously impact your migration. I am amazed at the IMAP folder structures people have build-up over the years, this can get really insane.
Update As of April 2011 this limit has been raised to 225 characters
The extreme on one side is a single folder of 40 characters, the extreme on the other side is a nested structure of 20 one character folders:
No more than 1800 IMAP folders are migrated
You are not likely to run into this limit, but hey, don’t underestimate the average user