At the end of the day, Microsoft is legally compelled to comply. As a legal entity, the focus of Microsoft's legal team will be on the legal position of the company, not the constitutional rights of the users, which are at the heart of the NSA PRISM program
controversy. More importantly, perhaps, is what happens if a legal order is issued for access to a specific customer's data by the government that results in other customers' data being locked down, simply because they happen to share the same logical space,
or may have been otherwise "a party of interest", yet committed no illegal actions - their only snag was that they were in some way logically related to the customer in question. Risk exists here. Each of us will need to assess the risk for ourselves, though.
I personally find the risk too great, even with the best of intentions from Microsoft. But for small to medium businesses that don't mind the shared infrastructure and don't want to carry the burden of employment of properly skilled staff, it makes sense.
I just wish Microsoft wouldn't push it so hard on their enterprise customers. I think their time would be better spent fixing the issues and shortcomings of Exchange 2013.