Last summer, I was a part of a team of five people (two ZB employees, three consultants) that spent more than a day migrating our hosted email to Mailtrust (a Rackspace company).  The migration cost thousands of dollars, the product was terrible, and it wasted tons of my time.

This evening I spent about two hours migrating to Google Apps for email, calendar, contacts and documents.  I did it entirely by myself, it cost nothing, and so far everything is about a thousand times better than the old system.

I hate to sound like a fanboy, but if Google offers a product, there's no point in looking elsewhere.