Cloud vendors offer content delivery network (CDN) services to compete for the video market. The user experience and the costs of providing the same video streaming service can vary when using different cloud CDNs. We emulate video streaming users in PlanetLab cloud to measure cloud CDNs including Amazon Web Service (AWS) CloudFront, Microsoft Azure Verizon CDN, and Google Cloud CDN. We leverage an approximated Quality of Experience (QoE) as a metric for evaluation. Our study finds that: 1) cloud vendors vary in providing QoE across regions; the video provider should assign a user to the CDN offering the best QoE at his location; 2) the QoE provided by one CDN can change over time; the video provider should adapt the CDN selection according to the real time QoE measurement; 3) cloud CDNs vary in scalability; streaming sessions may crash when there is bursty user demand; video providers should choose among the cloud CDNs that can properly scale; 4) regarding the cost, some cloud CDN is more economical than others given certain cache hit rate; video providers can minimize their costs by forcing free trial users to stream from the cheapest one.