I've been using Mint for over 4 months already, and I love it! Its "social" features (like comparing your expenses vs. other Mint users in your geographical area) are awesome. And I salivate just thinking about the amount of data that they collect!!! I agree that their sample is not representative, but if your research focuses on that top technologically-savvy strata of the population that spent a lot of time online, these data are perfect for insights on spendings, brand preferences, consumption of media and services, etc.
And today CEO of Mint shares some of these insights in the post on TechCrunch.
