We really love it for long trips. Get a couple of new kids movies, drive a thousand miles, drop them off at the nearest redbox which are now pretty much anywhere. We've even picked up new movies mid-trip. Can't exactly do that with a netflix membership.
Of course, what will be even more perfect is when netflix or a competitor finally starts "renting" online at redbox prices. I know I don't watch that many movies, so when I see a monthly membership, all I see is wasted money. But if they'd let me pay as I go, at redbox prices, that'd be awesome.