there are a couple guys at work who are into these types of background noise settings.. but they kept getting upset at what they found because after a while you can clearly tell the cutoff point. you'd hear a quarter drop at some point, then know exactly when it was coming later, and your mind would start focusing on that, and that defeats the point of the thing.

if you could make a mix of coffee shop sounds and then randomize them in a way that feels natural, that would be a hit ;)