I have included a link to a promotional page which is fairly typical of the many around the world. The project has been rolled out in about 25 countries so far. There has been a good take up, with a 60-70% retention from free trial to paid for service, possibly because of the low cost. It has been posible to keep the cost down by economies of scale as one set of questions can be used for all the countries.
With regard to effectiveness, this is a much more difficult area to quantify, we will never get the chance to compare the start and finish vocabulary level and understanding of English by users. Our take is that any extra reading/thinking in English is useful.