When I was at the iTMS, the only competitor that anybody was afraid of was Amazon. I'm certain that that awareness hasn't changed in the last few years.
I bet they do end user support way better than Google, though.
As for the Kindle app for the PC, Mac, and iOS, love it.
Amazon is capable of producing high quality products, but they really need to be careful about letting crap be released. It can do irreversible damage to your brand.