I have some ortlieb backroller classics and i absolutely love them. The fastening system cannot be beat, no bump at any speed will cause those to fly off the rack (which has happened with my arkel UB).
When i go grocery shopping i just leave the top open, and with the top open these things are HUGE. I can fit over a week's worth of groceries in the open panniers.
I have not had to use them in the rain much, but one day i did turn them into a cooler for some cold beers in the park, complete with ice. The ortlieb cooler pannier worked out well and kept my beers cold throughout the day. After i used them for this purpose i needed to wash them and they were super easy to wash.
The ortliebs were completely worth the $128 i paid for them (including shipping).