From the “official” info I got about the site:
Pex4Fun (http://www.pexforfun.com/) is a web-based serious gaming environment for teaching computer science. Pex4Fun can be used to teach and learn computer programming at many levels, from high school all the way through graduate courses. With Pex4Fun, a student edits code in any browser — with Intellisense — and Pex4Fun executes it and analyzes it in the cloud. Pex4Fun connects teachers, curriculum authors, and students in a unique social experience, tracking and streaming progress updates in real time. In particular, Pex4Fun finds interesting and unexpected input values that help students understand what their code is actually doing. The real fun starts with Coding Duels where students write code to implement a teacher’s specification. Pex4Fun finds any discrepancies in behavior between the student’s code and the specification. This tutorial instructs materials to equip participants with skills and knowledge of using Pex4Fun in teaching and learning, such as solving puzzles, solving Coding Duels, exploring course materials in feature courses, creating and teaching a course, creating and publishing Coding Duels, and learning advanced topics behind Pex4Fun.
Clearly, they exaggerate quite a lot (reducing CS to programming, assuming this would be useful for graduate courses, I didn’t see the specification part in the site unless this “specification” is just an english comment, etc) but worth a look anyway.