Anecdotally, it seemed like Hipmunk had nothing to motivate users to actually book through them. It was a great site to find what specific flight to take. The sorting by "agony" was great and I have never seen any other site have an interface as seamless as the sliders you could use to narrow down flights to specific windows of time. But once I had my preferred flight number, I booked somewhere else to get that mile bonus or whatever other incentive existed elsewhere.