I shop for groceries every two weeks (twice a month) so planning a menu definitely helps me with making a grocery list. I find that it also helps with not wasting as much food...but I'm not perfect and I just don't get around to doing it every time! If my DH goes with me it's a deal-breaker too...it's like shopping with a kid that wants everything in the store!
I love to cook and my MIL and I have a great arrangement. She lives alone and likes to have us eat with her, so we alternate...one night I cook and she eats with us, the next night she cooks and we eat with her. Very convenient since she lives next to us!