[Disclaimer - I'm married to one of the founders so very biased :) ]

I'm a huge fan of their approach. They're taking a hardcore CS approach (recommendation algorithms, machine learning, etc) to a space that rarely sees lots of tech applied. And they close the loop all the way to having great suppliers/product.

This is exactly what Marc Andreessen was talking about in his software eating the world article.

Really creative application of machine learning! Would love to hear about this as a paper on one of the ML conferences someday :)