This will be a big deal. People are a bit conflicted in their use of Sci-hub, so there's definitely an appetite for something that works without requiring you to support copyright infringement.
Very interesting way to get the full papers. This is how they do it: "We gather content from thousands of open-access repositories worldwide. To help us, we rely on a number of data sources, including PubMed Central, the DOAJ, Crossref (particulary their license info), DataCite, Google Scholar, and BASE. After we put all this data together, we in turn make it open for reuse via the oaDOI API: a free, fast, and very scalable way to leverage our data and infrastructure to support your own projects."
Really hope this becomes part of the mainstream scholarly communications infrastructure - it's slick enough to get people to actually use which is probably half the battle.