There's no question that flexible duct has revolutionized the HVAC industry. The advent of flex duct meant that contractors could dramatically reduce the amount of time and skill it takes to install a duct system. Not to mention that flex duct costs significantly less than other types of duct systems and is easier to store and transport.
As with any great product, however, the key to its optimum performance is proper installation. Too often, flex ducts are not installed properly, and they end up kinked, bunched up, undersized, or sagging.