EDIT: I ended up writing one myself: https://www.felixparadis.com/posts/arc-dot-io-review/
In a nutshell:
Arc is a peer-to-peer Content Delivery Network (CDN). Add Arc's widget to your website, your users contribute bandwidth, we rent it, and you get paid the proceeds.
A monetization method that doesn't involve selling or ads sounds enticing, but I can't find many reviews of arc.io and feel reluctant injecting someone else's JS code on my website.
The hackernews discussion about arc.io boils down to almost everyone saying they wouldn't use it, but some admitting it's a cool technology.