I much prefer the "dry-bag" style of backpacks that don't use zips (maybe for a small front pocket). I've previously cycled with many a brand of "waterproof" rucksacks and they always seemed to let in water through the zip in a heavy downpour. With my current rucksack, the top rolls down and then buckles together to form an airtight seal and that is absolutely waterproof and the contents would survive being thrown into a river. It's also extremely robust as there's nothing to go wrong apart from the material itself becoming worn through (not happened after years of heavy use).