I don't take the panniers off of the bike for groceries, preferring to load the plastic sacks (which I reuse for trash) into the panniers. Loading a full pannier onto a bike would, in my case, be a challenge.

I can get 2 regular bags into each pannier, plus I have a foldup backpack I carry for two more grocery bags.

I have panniers on 2 bikes (total of four). One set from EBay - and one set on sale from mail order.

Additionally, my wife has one pannier on each of her bikes.

Each pannier uses a different mounting system, but they all will fit onto a rack securely, which is definitely a concern to watch for.

I don't hit any of them with my feet while pedaling.