I'm a bit biased, but I don't feel S3 is the best possible fit for a frugal consumer's backup data. S3 is a high performance, general purpose solution capable

I'm a bit biased, but I don't feel S3 is the best possible fit for a frugal consumer's backup data. S3 is a high performance, general purpose solution capable of very low latency. Having that performance makes it more expensive than an archival specific solution, which tolerates latency just fine as long as high throughput is maintained. There's a reason why Mozy, Carbonite, SpiderOak, etc. all run our own data centers. I can sell a 100GB account for usable across unlimited computers for $50 per year and be profitable. That said, the market certainly has plenty of room for more backup choices, and any option to help people avoid data loss is a win. An exclusively Mac app seems like a good idea.