No matter what the manufacturer may claim about his panniers, assume they will leak in the rain! Treating the seams of new panniers is of course recommended, but protection of your equipment from inclement weather should go much further.
To begin with, line each pannier with a heavy-duty garbage bag (the 13-gallon size will do nicely). It’s not a bad idea to then load clothes and delicate equipment into separate smaller bags, just in case the large one should get torn.