I've just been recommended a free app called letyano (I am using it on my iPhone but I think it works on the other ones as well). If you are in South London this really works - no spam, no hassle - they have a load of coffee shops and restaurants (even a yoga place) and they all want to know what you think about their service - you answer just 7 questions (took me less than a minute to register and answer my first survey) and you immediately get a reward - a free coffee, or half price yoga session for me!