All interaction with the system happens through REST interfaces. Even our own UI uses only our publicly-available REST APIs.
There's a certain amount of free quota available. If you sign up you can try queries against public data sets like Wikipedia edits. Also it looks like the GitHub guys have been experimenting with analyzing GitHub data with BigQuery: https://github.com/blog/1112-data-at-github
I've just joined the team recently, but I really believe in what we're doing. I'd be happy to answer any questions I can.