PiCloud is a layer of abstraction above the IaaS providers you've listed. That is to say that it gives you software infrastructure to design workflows and distribute them across the machines provided by an IaaS provider. In some ways it's like Heroku, and in others it's another layer beyond Heroku (App Engine-like?). You architect your work around the PiCloud tools. I think PiCloud currently runs on AWS.

For me? Well, I'm a computational epidemiologist, so targeting science is exactly it's advantage. PiCloud allows the use of AWS resources fairly seamlessly from inside scripts, iPython notebooks, etc.

They're also very responsive to questions, and their package is far, far easier to use for the things I use it for than trying to build something myself via AWS, Rackspace, etc.

Disclosure: They also gave me a $500 research grant.