I use two different sets of panniers.

For touring, I have Caradice Super C's. They are large and made of cotton duck. It's a canvas like fabric that is durable and repairable if it wears a hole. They aren't considered waterproof, but water resistant. However, I have heard they don't let any water in, and are always dry. They have only been out in the rain once and everything stayed dry. High quality and well constructed. Probably the last set of panniers that I will ever own. Only con is the mounting clips. They can be a pain with my big hands. but they are infinitely adjustable and will never unclip and/or fall off. I like the Ortlieb pull handle system better, but not enough to switch.

For commuting I use the Nashbar waterproof panniers. They are light, cheap, and extremely waterproof. I have been using them for two years, which is much longer than I thought they would last. I've worn a few tiny holes in the lower corners from throwing them around fully loaded. I cant say enough good about the durability of this set. They have hooks and velcro to hold them on. Again, not high quality, but they work and the panniers stay on as long as you use the velcro. If these wear out, I'll buy another set. They are perfect at the price point.