For the last 2 years I have commuted about 30 miles a day using the Nashbar waterproof panniers that are linked above. I carry clothes, lunch, textbooks, and one or two laptops. Some days see them with 25-40 lb loads. They have lasted and seemed to have been a great investment. Some have had bad luck with them, but mine have been great. And not once have I had to poke holes in them to let the water drain out, although once my lunch exploded inside, and it kept it there.
When I tour I have a full set of Carradice Super C panniers that seem to be indestructible.