If you're going to be biking long distances in the rain it's a good idea to get waterproof panniers. Unfortunately these are expensive and I don't think they're easy to find on the used market. The cheapest I know of are the Nashbar panniers:
http://www.nashbar.com/bikes/Product_10053_10052_172282_-1___ (they're usually $50 for a pair, don't be fooled by the "sale" lingo)
We own a pair of these. Their only real advantages are they're huge and cheap. Disadvantages are they're annoying to attach/detach to the rack and annoying to close. Based on the material, I'd guess they'll last for about 5 years of regular use before they're no longer waterproof.
When I had access to wholesale bike accessories I bought a pair of these:
http://www.axiomgear.com/products/gear/bags/tempest-series/typhoon-aero-dlx/The Typhoons are lighter than the Nashbar panniers (only really a concern if you're touring) but at the same time made with super heavy-duty material. They have better quality rack attachments that make it easy to attach & detach them from a rack with one hand, plastic D-rings for attaching shoulder straps, and a simple roll-top that's easier to close (there's also a creative way you can carry extra-long items like a yoga mat by cross-buckling). The only downsides are they're a little smaller than the Nashbar panniers and the hooks can get caught on each other if you carry them together (there's an easy way to fold the hooks up to prevent this, but when you forget you'll spend 5 minutes trying to get them undone).
They also cost a lot more than the Nashbars. I paid more for them wholesale than the Nashbars cost. I suspect these will last me at least 10-15 years, though. I had a Jansport backpack made from similar material that lasted longer than that, and the panniers come with extra hook hardware in case something breaks.
When I was looking at prices I seem to remember most other quality waterproof panniers costing more than the Axioms, so I tend to view the Nashbars as an interesting outlier.
