Similar to Khan Academy (and a lot of games), the site implements achievements to reward one’s progress. Mine can be seen in my profile page. It didn’t take too long to finish both the lessons that are currently available. It does look like they plan to implement more exercises soon, so that’s good news.
The hands-on approach works great for programming languages, and it is implemented pretty nicely in this site. I’m looking forward to see new languages in the same format — this is the kind of thing I’d happily pay for.