It works quite well (at least on me). Responses usually go from "well, I was already feeling like one of those so might as well get some points" to "why would I want a Frappachino when it's so cold out"? I assume their algorithms are under heavy development and are basically random A/B tests at this point, perhaps primed with some basic data and stars added as an enticement.
In the future, I could see it evolving into something where it sees what the weather is, looks at what websites I'm visiting (via data brokers), analyzes historical trends, extrapolates my likely mood and finally uses all of this to figure out what sugary comfort drink I'm mostly likely to bite at. Perhaps I should take comfort in how utterly predictable I am and how easily open to suggestion...
On a similar note, if you haven't seen it; https://www.ted.com/talks/zeynep_tufekci_we_re_building_a_dy...