I think it depends on how saturated it is. I think, from memory, that the charging speed goes down if you have someone else sharing the post. I've certainly seen my charging rate drop to 3-4kWh when another car is charging alongside at a local Tescos. That might not be the case at every site though.
I would disagree that it's a waste of time. If you're only popping in for something, then sure, but if you're doing a big shop or spending a reasonable amount of time then at the very minimum you'll get the distance you travelled to get there effectively refunded, which seems ok to me? Point me at which supermarkets give you some free petrol when you turn up and I'll reconsider my thoughts about it.