[TomFrost]

ECS is free in that you pay only for the EC2 nodes running your containers -- there's no need to host ECS or do scheduling on your own hardware to use it. It's also Availability Zone-aware right out of the box, making sure the distribution of container instances is optimized for durability. Finally, it's fully managed. No one needs to maintain or upgrade your ECS implementation.

Granted, there's a lot of advantage to building on top of an infrastructure that can be installed on any hardware from any provider. However, we're not talking about rewriting your applications if you need to move away from ECS; it's all still the same containers. Going from ECS to Mesos or Kubernetes when needed is a matter of writing new config files.

It's a very attractive proposition for small teams on AWS who are trying to spend minimal time on ops.