Most of the waterproof ones are one big compartment and tend to be lighter than panniers with lots of pockets compartments and doodads.

Me, I like the one compartment approach and prefer to organize stuff with stuff sacks or ziploc bags. Some like lots of pockets.

I am happy to use the inexpensive waterproof ones from Nashbar or Performance. They have held up well for me on a bunch of long tours including a Trans America. I have since started packing lighter and lighter and now just use waterproof stuff sacks with no panniers. I think for me at 35 pounds or more I like to use 4 panniers, but at 25 pounds or so like to use front only ones. Much lighter than that and I go with waterproof silnylon stuff sacks.