Datastax offers so much as a managed Cassandra platform, and they've emerged as the primarily leader in this space. The training materials and documentation alone are awesome contributions. Datastax has contributed so much to the open source tooling, and their technical blog rocks. Datastax Enterprise is really what you're looking for and should really consider when starting up a cloud app with Cassandra. The integrations with Spark, Opscenter, and Hadoop are what makes Datastax Enterprise worth it, not to mention the service level.
We were looking to build scalable, client-facing APIs, and Cassandra worked well in our implementation. Datastax really completed the picture.