One thing that Kayak is doing better than Hipmunk is multi-airport things. I.e. We have here in Silicon Valley SFO and SJC. I usually prefer SJC, but can fly from SFO too. Some flights are cheaper or have less connections if you depart from SJC but arrive to SFO, or vice versa. I couldn't find how to tell Hipmunk that it is OK (or not OK) to use different airports for round-trip. Kayak does it very nicely. It also seems pretty hard to say "I like these dates but wouldn't mind to go day earlier-later if that gets me couple of hundreds of dollars back".