Hmm. Filed in my 'Worth investigating' folder. Of course the downside must be that you would lose any sales ranking you would have acquired at Amazon/Smashwords. Where do Gumroad make their money on this?
Selling on your own web site or blog shouldn't change your Amazon or Smashwords rankings in the least, unless you start selling far more on your blog than on Amazon or through Smashwords and then you wouldn't care because you'd be earning so much more income.
Gumroad takes 5% plus 25 cents on a sale.
If you sold a $3.99 ebook on Amazon (and it was small enough to avoid any Amazon delivery fee) you'd earn $2.79
You'd earn $3.39 on Smashwords, and $2.39 through its retail partners.
If you sold the same $3.99 ebook on your blog/web site using Gumroad you'd earn $3.55
Just an update note:
If you use the free version of Gumroad, you will have access to less features, and our fee per sale will be 8.5% + $0.30 (USD).
The hellowebapp article looks at Amazon vs Gumroad vs Leanpub. It's an interesting article if just for the way she went about studying the results.
There are other payment processing services entering the market all the time for the sale/delivery of digital goods. Gumroad is only one of them. I use them because I've been with them for several years now and haven't had any issues nor reason to consider switching.