It depends on what your definition of "truly successful" is. I recently started working on a .NET app, but I was thinking about switching to Java when I heard about Amazon's Beanstalk offering. If these guys can come close to replicating that for .NET, I'll take a good look at them.

At the end of the day, though, they have an idea, and have convinced investors that there is enough of a market for this type of service, so good luck to them!