If you go to work on a bike or if you do long distance bike touring, bike panniers are a must. You only start paying more attention to the quality of bike panniers when you are unfortunate to have to ride under rain and you get to your destination with the contents of your panniers all wet.
That is what happened to me. I´ve have several cheap panniers over the years. The panniers I did my Bracknell to Paris ride were purchased in Aldi for, I think, £15 or less. I still have them, but after that ride they took a beating and I would not use them again for such a long ride.
Ortlieb Panniers are among the best money can buy. It may look like they are very simple as they are basically just one big compartment with a little side compartment on the back inside the pannier.
What is so good about them is, of course, the quality of the material they are made off, the easy rack attachment system and the fact they are extremely water proof.
During my last tour in the Isle of Wight I had to endure quite a bit of rain and even a hail storm along the way. I had not only clothes in them, but also sensitive electronic equipment that didn´t like water very much. Not a single drop got through them and I have even heard experiences from people that actually left them in a flooded field with a laptop computer inside and everything was fine and dry.
There are many reviews on the internet and YouTube, so I´ll just borrow one…
If you can afford them, I highly recommend.