My Take
Free Rice is a great tool for students preparing to take any of the big standardized test (in fact, the developer created it to prepare his kiddo to pass the SAT). My students love the fact that it's easy to use and they especially love the fact that they can feel good about the social activism element to this game. I love that the app uses adaptive learning technology, offering more difficult words when students answer correctly and offering easier words when students answer incorrectly.
How I Use It
I highlight FreeRice on my class website and offer it to students as a study aide for test preparation. We spend a few minutes looking at it on the day I introduce it, but for the most part students use it on their own... like when they finish a test early or are at home and want to improve their skills.
Full disclosure: I am an addict! It can turn into a bit of a time sink for me... I can't stop playing!