I think there's a grain of truth to the companion plant stuff, but I also think there are a lot of additional factors in play that can affect the outcome, as Bob said. If I plant marigolds, it's more because I like them than because I think my peppers (or whatnot) will benefit.
On the other hand, I"m trying mint as a companion plant in my veggie beds this year, to see if I can keep the bunnies from smelling the delicious produce. I just lifted the row cover on the tomato bed and discovered the peppermint is as tall as the tomatoes (we're talking 2 feet tall!), time to give it a haircut. The spearmint ('Kentucky Colonel') in the main veggie bed is much less thuggish, so I think it'll be OK with cukes, beans, etc.
I didn't realize comfrey had such a long tap root. Guess I will try to work around my one big clump of it when setting up my new raised bed rings... I was thinking I could move it over about 18", but I'm not digging that deep!