I agree with the others that pannier covers are a waste of time. We have some bright yellow ones so we put them on for visibility when weather is yucky, but not to protect the panniers from water at all.

We just use the regular plastic grocery like you get every time you buy groceries. We put each stuff sack in its own plastic bag and pack it that way. Unless we are passing through the desert where I know it won't rain (like now) I just keep everything in my panniers wrapped in plastic at all times. It is quick and easy and I know what is in every bag. If I get caught in a downpour, I don't have to worry about stopping to prepare.

I hate using the great big bags (except for the tent or other big items) because they are too hard to protect on a long tour. If you are only going to use them when it rains, where do you store them otherwise? And how do you replace them in other countries? We have one big bag that John uses for the tent at times and he treats it with the utmost care - to replace it we would have to somehow find and buy a whole box of bags and then only use one.

the regular grocery bags are easy, cheap,a nd easily replaced when they wear out.