I was reading an article from Ted Weldon at Carryology regarding the necessity of having 100% totally waterproof packs, panniers or bag and brought up several points to consider for bike touring.  

I have used my set of Axiom Cartier Panniers for approximately ten years now.  I used a can of Camp Dry to provide more water resistant protection than the Denier material would have provided by itself.  I also use some water resistant stuff sacks in three different sizes which have not only served to keep my goodys dry but to help keep my panniers more organized as well.  I like the mesh exterior pocket and zippered rear pockets as well as the zippered pocket in the cover.  The elastic cord which is missing is the only part of my panniers which have simply rotted and disintegrated a year ago.  While riding in Portland I noticed a friend had invested in some very large, obtrusive waterproof panniers and considered the rainy climate to be the basis of that purchase.  While visiting with some friends I decided to leave my bike parked out under the rain to see how long it was before it soaked through.  After leaving it out overnight I had nothing to complain about the following morning as my stuff sacks inside the panniers were barely dampened by the moisture.  

It has been on very rare occasion that I forged through a torrential down pour when the rain is so heavy you need clear lens riding glasses just to be able to see well enough to continue riding.  With bike commuting that's fine as commuting usually necessitates much shorter trips than while touring, in which case simply waiting it out works just fine. 

Here is a quote from Ted's article to give you an idea of what he has to say on the matter. 

A link provided previously in this post is worth checking out, I got a good chuckle out of it and enjoyed his ability to relate common sense decision making over imagining worst case scenarios to worry about and perhaps either over spend or simply find yourself carrying too much weight in those heavy, ugly waterproof river dry bag looking sacks that some companies are calling panniers.

After ten years of riding through mud, sleet, snow, plenty of rain and intense UV exposure which by the way probably contributed more than anything else to the elastic cord wearing out.  Axiom's Cartier Panniers as shown at right differ slightly than my older set.  The mesh exterior pocket is now enclosed and the material looks to be of better quality Denier than my older pair.  Still a very well reviewed set of Panniers that I will purchase again.  At less than a hundred dollars for the pair it's a no brainer.  Other features I like about these panniers are the size and shape.  An angle front allows for ample heel clearance even on my mountain bike for commuting, the attachment system provided on the back of the bag for attacking to a rear rack is versatile enough that I have been able to attach them to three different racks all designed by different companies other than Axiom. 

Kiwi Camp Dry has been around a long time and has become a popular product with all sorts of outdoorsy folks.  Camp Dry has helped to make my tent, backpack, panniers and riding coat much more water resistant than if I had not used anything at all. 

For the very modest price and ease of application it has become a standard practice to treat new bike touring and commuting panniers and bags when they are brand new and prior to first time use. 

For under ten bucks it's worth the small investment to at least give it a try some time, you'll be glad you did.