One of our assignments for Human-Computer Interaction was to carry out a formal usability evaluation of a game. Unsurprisingly my group chose a mobile game, X-Construction Lite, and did a study on it: first individual evaluations using a cognitive walkthrough (AKA "having a play") and referencing Nielsen's heuristics to find and categorise specific issues.
We then did a lightweight usability study by ambushing students outside the campus library and videoing them playing the first level of the game, in exchange for cake (pear tart from Real Patisserie, if you were wondering). Interesting exercise; it's a good game that seems to have gathered nearly 70,000 reviews in the marketplace (and to which we're all now addicted), but our testing showed up some consistent and fundamental problems with the gameplay.
We've submitted a full report, but you can see the presentation we gave summarising the project on slideshare here. Credit due to other team members: Andy Keavey, Queen Atifa Ododo, Mariana Rojas-Morao and Merve Yildirim.
I can't help but direct you to a photo of one of the other groups, showing off their Plants vs Zombies review in appropriate attire.