Hey, my buddy Dan Holmes posted an entry about moving his company to BPOS. Definitly worth reading.
Quoting from the above:
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. I’m actually blown away that the company seems to have done so much right the first time around. I know you know what I mean! I heard about Microsoft Online for the first time 18 months ago, and Microsoft has delivered. It’s competitors can talk the talk, but Microsoft brings it across the finish line. So Microsoft Online is absolutely worth a very close look by any enterprise wanting to improve levels of service and reduce costs simultaneously. Just be sure you’ve done your due diligence to make sure you’re subscribing to the categories and levels of service to meet the requirements of your organization and its users.