Readability aren't the first to do this. In fact, several ebookstores announced a similar feature within months of the Kindle's launch back in 2007 and calibre has been able to email stuff to your Kindle since mid 2009. But it is nice that they've added the option.
But there's something that puzzles me. Emailing content to your ereader is a clever and useful feature. Why haven't any of the other ebookstores copied it? At this point I'm beginning to think amazon have a patent on the idea. I haven't found it yet, but i can't think of another reason not to implement this feature.
Also, why haven't Amazon expanded this feature beyond the Kindle? I'd love to be able to email ebooks to my various Kindle apps, but I can't. Of course, most of the apps are running on devices that also have an email client, so it's not important. But it would be nice to cut out a step.