Comments were negative, but still, I learned about Hipmunk through reddit and now do check Hipmunk (and many others) when I need a plane ticket.
But what really gets me is the UI. It's my favorite one out of all the websites I know.
Hipmunk's UI is WAY better for me trying to pick flights. I don't care that I might save $10 or whatever, as much as avoiding 8 hour layovers, or leaving at 5am. You can't easily tell any of that with an interface like Momondo's (which is just a pretty version of every other travel site).
You can put a max on travel-time and choose which part of the day that you want to travel in.
Those sidebar features are the same on every travel website, except Hipmunk. The linear bar graph display of flights is, for me, infinitely superior than a data tabel that I can drag sliders around to change the wall of text. It takes me a glance to see that there are great options or shitty options on a given hipmunk search, even on my iphone. Momondo and basically every other site require significantly more cognitive work on my part to piece apart their text boxes full of information to figure out if that flight is a good fit for my needs or not.
(Monmondo/Skyscanner) 162 vs 177 (himpunk)
And the UI just doesn't matter with these sites.. do search, ignore crazy prices, select the legs that suit you best..