If you invest into a cloud service and they go bankrupt the next year, what happens to company data? Yikes!!! One rule-of-thumb (as you mentioned) is to do the research on that reliability of that cloud service!

Funny thing though, people are so worried about what security services in the cloud has (like their data center security is so much better). I would bet that cloud security from Amazon for example far better than most mid-size company local security programs (if they even have one...)! What people should be asking is, will cloud security be better than the security program they already have! :-)