I respect what Hipmunk is trying to do, but I really doubt they are filling voids outside of "this" community. I find it much harder to scan, slower to load, and having lower quality tools for finding alternate (or cheaper) fares.
It's interesting, considering they will provide round-trip flights and list the London to NY flight(s) as a separate item, which would make me think the data is there.
Hopefully Priceline will learn some improvements from Kayak and not try to force changes the other direction. Kayak has a great UI overall, but the explore interface is pure awesome: http://www.kayak.com/explore/
Visually that kind of interface is very appealing, but fares may be misleading... once you start the booking process, you will end up with completely different numbers. Typically, a featured "as low as $" air fare is a red eye, 8 hour connection time etc. I was going to output specials myself too, but then commented them out - as they hardly ever apply.
That being said, Kayak does have a great UI, and their iOS apps are eye candy.
I was getting ready to say almost the exact same thing -- thank heavens that Hipmunk came when it did, or that acquisition (if soured) would have left a very large hole in my travel-making tools.
I don't particularly like Kayak, but as a matter of effectiveness, there's no doubt that it was pretty much king of the hill for a good long while. Since Hipmunk's inception though, it has definitely been the 'go-to' place for travel-planning, and beats the nearest competition (that I've used) by a mile.
I would also venture to say that this was perhaps a timely exit for Kayak, as the more and more traction Hipmunk sees, the less off the Kayak founders would have been for an exit.
Congrats to the Kayak folks I guess, when they got into the industry they really raised the bar.
Before Kayak I remember having to use Expedia and Orbitz and just having to accept their crappy UI because there simply were no alternatives.