Yes, Hipmunk is very usable once you figure out how to use it, but after being trained by every travel website on the planet for years it is not very easy to use at first. I agreed with most of the authors points.

Last time I took a trip I tried out Hipmunk and was very confused about the picking my return flight separately. Once I figured it out, it was pretty cool. I still used Kayak though.

Kayaks ability to specify departure and arrival times is just too convenient if you're trying to work around a schedule. I think people like picking flights separately on Hipmunk to solve that exact problem and it's much simpler to do that on Kayak. Maybe I'm wrong, but that's why I like Kayak better.

As far as timing out searches goes, flight prices really do change every few minutes. I've waited an hour to buy a plane ticket and had to pay 20 more dollars. Kayak lets you save searches so at least you don't have to enter all your information again.