Over my past two positions as a User Experience Researcher, I have used Qualaroo on cancellation pages to understand churn, on pages with new features to gauge reactions, and on common landing pages as a way to link to a longer survey.
Qualaroo makes targeting and traffic allocation much easier. It get a better response rate than Usabilla. It has more limited survey ability than products designed specifically for surveys (SurveyMonkey and Qualtrics), but those packages do no provide pop-ups in the standard package.
Compared to other products, Qualaroo pop-ups are easier to spot. If you want something very unobtrusive, this might not be the product for you. However, I have never heard customers complain about the pop-ups and they receive great response rates.