Oh yeah and its really difficult to stay dry from the rain while not overheating from sweat. Lots of products claim to be waterproof and breathable but often in rain you will either get wet, or sweat. So you may need to bring a change of clothes. (if you normally ride in what you work in)
If you ride in rain often, and if you want to keep your stuff dry, Ortlieb panniers are a good investment.