Like most said, I prefer eating smaller ones...like 2-3 lbr's ideally. I won't keep one over 6lb's if they will swim away (in zone 20), nothing over 4lbs in back lakes...although i don't really fish back lakes for walleye since the slot came in as I like to eat them and don't catch enough to be that picky. I do think that low end of the slot (15.7") is silly though...that's a prime eater in my books. I don't eat the big ones more due to the contaminants' issue than the breeding issue. I'm not sure an old 15lb sow is really a prime breeder...just wouldn't want to eat one due to the amount of time it's been swimming in polluted water. I have eaten an 8lbr once as it wasn't going to make it. It tasted fine...just do like pdubya said and trim away the dark meat along the lateral line...it tastes awful. As far as the contaminent build up, I believe it's more in the fat of the belly then the dark meat.
Don't forget the cheeks. mmmm