Chewy?
My experience with enough brands of panniers to form an opinion of one over another is limited so I'll let others comment on that.
But generally, remember as you add weight to the back of the bike, the stability will also change. If you load up the rear and notice the bike feels squirrely, rearrange the load to keep the weight forward if possible. If you have a ton of weight on the back and not much on the front, you run the risk of the tail wagging the dog, as it were.