Sourcegraph CEO here. It is for reading code. It gives you the full power of an editor (jump-to-def, hovers, find references, advanced search, symbol search, cross-repo references, etc.) for any repo at any commit on GitHub. It supports Go and Java right now, with some more languages in beta.

If you find yourself reading code on GitHub a lot, or cloning repos to open and read in your editor, you'll find it useful. If not, then you probably won't find it useful, but you might like our Chrome extension, which adds these features inline on GitHub.com (https://chrome.google.com/webstore/detail/sourcegraph-for-gi...).