I believe Docker is great for mass-spawning tons of similar isolated containers, but feels more like an obstacle than help for other isolation/virtualization needs.

I've tried to use Docker to just isolate non-distributed ("singleton") services (in particular, virtualizing a single Puppet Master) and found it just staying in my way and complicating things more than they should be without any gain. But if I'd need to, say, launch a bunch of redundant servers or provide a PaaS-type hosting - I'd probably use Docker.

That is, I guess, due to a hype, Docker just accidentally got marketed outside of its problem domain.