Here is a link to Dan Holme's articles on his experiences with Microsoft Online Services.

The report card:
"Final Report: "A" for Effort & Service; "A-/B+" for Features – Absolutely Worth Exploring
What Microsoft has attained in this first release is an extraordinary, robust, enterprise-ready service. I don’t expect there to be many fundamental problems (security, reliability, availability, performance) based on my experience thus far. ": 

His migration experiences: