How important are gaskets compared to the cooler itself?

A tight seal helps, but the cooler’s insulation matters more. A cheap cooler with a perfect gasket still loses to a rotomolded cooler with a mediocre gasket. On high-end coolers, a bad gasket is a real problem, but on budget coolers, even a great gasket won’t save you. Don’t buy a cheap cooler just because it seals well.