Honestly? Pretty fucking awesome if you get it configured correctly. I don't think it's super useful for production (I prefer chef/vagrant) but for dev boxes it's incredible at producing consistent environments even on different OSes and architectures.

Anything that makes it less painful for a dev to destroy and rebuild an environment that's corrupt or even just a bit spooky pays for itself almost immediately.

In my experience, very, but it's also not magic. Being able to package an application with its environment and ship it to any machine that can run Docker is great but it doesn't solve the fact that modern deployment architecture can become extremely complicated, and Docker adds another component that needs configuration and debugging to an already complicated stack.

It's a way to provide standard configuration for your programs without one configuration interfering with another.

Honestly, almost all alternatives work better. But docker is the one you can run on any system without large changes.

I think they’re really useful, there are alternatives that I think have feature parity at this point but the concepts of containerization are the same