Brewpubs and breweries sell the jugs for a flat price and fill them up straight from the tap, usually for somewhere between $6-15. After it's emptied, the growler can be brought back to the same brewery for a refill. (Unfortunately, government labeling laws don't allow breweries to refill growlers that aren't their own, so you can't just buy a generic jug and tote it from tap to tap, although this rule varies by state.) Usually the cost of filling the growler is much less than the price of four pints by the glass, so buying beer this way saves money and reduces waste.
A full growler should be kept in the refrigerator and consumed fairly quickly. Like wine, once opened the beer will keep for two or three days. For even quicker consumption, bring it to a small gathering as an alternative to the usual six-pack.
Do you buy beer in growlers? Any brewery recommendations in your area?