At Xamarin, we’ve been using GithubSharp to access the GitHub API for some time. We ran into some issues with it, however, because the JSON serializers in .NET are buggy as hell (one had trouble deserializing a simple dictionary; another couldn’t handle large payloads). Therefore, we forked and branched the project and made it use ServiceStack.Text, which works nicely (and has the added effect of working on Mono).
Note that our branch is for version 2.0 of the GitHub API; the author of GithubSharp, Erik Zaadi, is now working on GithubSharp for GitHub’s 3.0 API, using ServiceStack.Text.
