RancherOS is plenty interesting. What sets them apart from the other micro Linux' like CoreOS is that it's really small. It's nothing more than a kernel with a few containers where the Linux services are running. The rest is meant to be userspace.
At first glance everything is really small. I think the presenter say 19GB. However, since the project is so young it's going to gain some weight as they get to the balance point between size and needed functionality. It's also very incomplete as the docs indicate the upgrade facility is not implemented yet.
One interesting feature or omission is the persistence. The entire OS is run from memory and persisting anything requires formatting the drive. So it's easy to conclude that the documentation and the feature set are also in flux.
The rancher project is interesting and functional. The last time I examined it I was pleased to get things going. It functioned in places that Deis did not. My guess is that Rancher wants to own more of the stack so I wonder whether Rancher is going to get the resources it needs to make progress while they are off capturing the OS market. While Docker is playing in that space Kubernetes and Mesos are not.
CoreOS still seems to be a solid bet.