I have used hipmunk a handful of times and it is dead simple to use. I do have a few problems with it: 1. There have been a few times that I have found a flight only to click through to another online flight aggregator (orbitz etc) to find they don't have the rates specified on hipmunk. Very frustrating. 2. I hate, hate, hate dealing with other aggregators (again..orbitz etc). I don't know if they do this already or…
#1 We do our best to mitigate this. Unfortunately, it's also the nature of the business. Availability is constantly changing. What's really mindblowing is that Orbitz will often have flights in their own results that will error out. This isn't specific to Orbitz either, everyone has to deal with this. #2 This is a big priority for us. We already have many direct deals, and more are on the way.
It's not just changing inventory though, I've had the same experience repeatedly over multiple days. hipmunk quoted me an itinerary, sent me through to (I think) US Airways to purchase it, and US Airways said it was no longer available and here's the same thing for $200 more. Gave up, tried again a few days later, hipmunk is still quoting the same fare.
Only happened for that one fare that one time, though. And I do love hipmunk and use it regularly. (Far more for speculating about places I'm thinking of going than for actually making bookings, I'm afraid).