I haven't used Cloud66, but I've seen them come up a few times. I think the biggest difference is Rubber has templates for common pieces of software and deployment scenarios. Since it's based on the role-centric system Capistrano provides, you can do things like set up several "app" machines and then a "web" machine, which runs HAProxy and automatically routes requests over those "app" instances.
But, I think we fill that niche between hand-holding and trusting devops/sysadmins to do what they think is best. We try to handle the monotonous tasks for you (configuring DNS, setting up backups, configuring load balancers, etc.) out of the box, but give you a great deal of flexibility to influence that. Almost everything is bash and ruby with a healthy set of utility functions.
I admit, it occupies kind of a weird middleground. If you use Heroku because you're not well-versed in sysadmin tasks, there will be something of an uphill battle (but a great learning opportunity). If you want to outsource everything, it's not a great fit. But, if you want a lot of flexibility, have unique app contstraints, want the abilitity to switch betweeen cloud providers and not deal with everything from scratch, it's a pretty good fit. I've personally used it with EC2, DO, vSphere, Vagrant, and leased hardware. It's nice being able to treat them basically uniformly.
