Genuinely curious, who is the audience for this? Large iOS dev shops who want hardware to run builds on and don't mind that you need to reserve them for at least 24 hours at a time?
Well it's anecdotal, but yes, our company fits directly into that space. You might ask why we don't want to just have the hardware in-house, but since we're developing Software as Medical Device products for iOS, we'd rather qualify Amazon as a critical supplier than try to operate the activity ourselves (with all the controls that come with doing so).
Large shops that already run on AWS and where iOS/mac is a small part so they'd rather dish out a few $100k for someone else to manage and have it homogeneous through their other systems.
And it's a standard service that any of their developers can access as needed without needing to either setup their own Macs as a service in a lab or distributing standardized Macs for testing/builds/etc.
For a small shop it probably makes more sense to just buy the Macs, especially if they do a lot of iOS/OSX development.
My employer really wants to use this. The IT department dislikes supporting Apple hardware and software. But...we have a product that needs to end up in the Apple App Store. They tried to convince the iOS engineers that it would be fine to work in a virtualized environment (and cheaper), but we really were not convinced that would be workable in the long run. I was willing to give it a shot, but the effort eventually fizzled out, likely over Amazon's costs.