Here is a waterproof pannier for the most discerning user, and it has a couple of advantages over the trusty Ortliebs.
Firstly the fitting system, the Arkel Cam-Lock. Pull up the handle, it opens the cams. Release the handle and the cams firmly lock on the rack tubing. No need for special inserts. The Cam-Lock will automatically fit rack tubing of 8mm to 15mm diameter. It fits the vast majority of racks on the market and is made of solid aluminium. Details are all here.
Second, the 32's and the 48's have some exterior storage, which you will appreciate if you have ever made the mistake of putting damp items in your extremely well sealed waterproof bags. One damp thing just makes everything damp. The Dolphins have an exterior pocket and a sleeve behind it which is ideal for maps and wet socks, and there is a mesh draining panel at the bottom. The 48's have a couple of side mesh pockets.
The Dolphins have a roll top which is held tight by a velcro strip and a couple of snap-lock buckles so its sealed but really quick to access. There is a sturdy handle and D-rings for a shoulder strap. Finally the back plate is extremely strong and resistant to bending.