Actually it's much better than hipmunk. I used to be a fanatic user of hipmunk. But once I tried google flights i immediately was amazed by its speed, browsing different dates is super fast and easy.
Google Flights' instantaneous search allows you to freely explore the option space opening up new ways of travel planning. Free for spring break and don't know where to go? Set your travel dates and pan around the map looking for deals.
Kayak doesn't even come close to this. At best, you can set the month of travel; useless for most busy people.
As with all things Google, it's an order of magnitude beyond what hipmunk does. If you keep the window open long enough you will actually see flights disappear if they fill up.
I still think the hipmunk interface is more humane.
