Well, I have several clients that have come to me and wanted to be moved to the gmail option. I have moved several "live" domains to the google option.
As long as you setup the mailboxes before you change (or soon after you change) the mx records, you truly don't have any down time. Granted you may have to check the old mailbox for the last bit of delivered mail.
One thing you'll find is gmail doesn't allow you to setup "forwarders", with the exception of setting up a mailing list or a "group".
For example if you have a forwarder: "sales" setup to forward to your sales department, google calls that a "group", and that forwarder would be established with google.
If you have 1 user that has multiple addresses, for example, I do a sheriff's department, each user has a login John Smith Login: jsmith
Since John Smith is the sheriff, you're able to setup in his user account to accept mail also sent to
So John Smith will login as jsmith, but can accept email as
But the traditional "forwarders", aren't truly forwarders with google.
I hope this helps, and if not I'd be more than willing to chat with you privately or answer any questions off the forum.