Remember, sous vide cooking consists of holding water at a specific temperature, and immersing vacuum-sealed food into that water, cooking it right up to the water temperature—and no more.
Not long ago, we tested the SousVide Supreme, a $450 machine that does this pretty well, and is "affordable" compared to a PolyScience precision water circulator. The basic premise is that you can keep water at a relatively fixed temperature in pretty much any insulated container, so why not use a cooler?
The tester, Chef J. Kenji Lopez-Alt—whose superior cooking skills and experience are in no way called into question here—demonstrated that a cooler could be used as a sous-vide device when bringing tender meats to a pleasing internal temperature (around 125º F) before searing, and for holding cooked food at a nice warm serving temperature. I buy that.
But even Chef Kenji says that there are places you can't go with the cooler method, namely, "the ability to tenderize tough pieces of meat," and "the ability to cook vegetables without loss of flavor." Those two are big benefits of sous vide, as is any cooking practice that requires you to hold food at a particular temperature in order to ensure that all bacteria has been killed, without destroying too much of the food's own flavor by overheating. I wouldn't necessarily trust a cooler with that task, either.
The cooler sounds like it makes for unexpected fun in the kitchen, and a handy backup for people who already know what they're doing. But it's not a replacement for the SousVide Supreme. That's not to say there's no replacement for SVS. I've seen a few hacks involving slow cookers and temperature controllers that do a job that comes pretty close, and I may test some of those out. I'll keep the cooler method in mind, too, but for now I'm leaving my beer in it. [Serious Eats]