I have a set of Seymours (largely cause the LBS didn't have the Cartiers in stock - I happen to like the name). Seymours will handle 5 lbs frozen chicken, about 4-5 lbs frozen veggies and some other misc groceries per side. They also handle 3-4 hardback books per side with room to spare. Rain jacket, rain pants, a pair of jeans, socks, a long sleeve shirt and underwear should easily fit in one bag, despite my rain jacket not packing well. They stay on well, even if I forget to fasten the bungee cords. I don't tend to jump curbs or aim for the biggest potholes tho.
They are listed as water resistant, but in practice, I'd class them as not waterproof or water resistant. If I want things to stay dry for even a few minutes, they need to be in a plastic bag. I have had things get damaged in the rain when not protected.
They would not provide good shock protection to a laptop, and I'm not sure a laptop + protective sleeve would fit inside one of my panniers. If I were carrying a laptop, I'd use my backpack, since it's designed to protect it.
I don't expect the Seymours to last for more than about 5 years. Some of the metal is starting to rust after 1 year. The seams aren't reinforced. The back plate bends if there's more than about 15lbs in a pannier. The plastic coating on the hooks has separated away from the metal, but still works to keep clattering to a minimum. It doesn't protect my rack from scratches tho. When I wear them out, I'll replace them with a pair of sturdier (and more expensive) panniers. I *will* have gotten my money's worth out of them, but IMO it's better to spend a bit more when I replace them so the replacements will last longer and be better suited to how I use them. (for reference, I've had 3 backpacks over the last 20 odd years - one is still usable but too small for me, one wore out, and one is brand new and should be good for another 10 years. I'd want panniers to be similarly durable.)