My mom's mother lived with us the entire time I was growing up.
She and my father did not get along at all. He must've had the patience of a saint. Actually he had his own escape den in the basement - fridge, bar, beer, TV, girlie mags, etc...I'm sure that helped. But it sure didn't do my parents' marriage any good.
And honestly I don't think it did any of us any good. Though it seemed like an ideal plan - Grandma did the cooking, cleaning, laundry, took care of me when I was little, she was a bitterly unhappy woman and made sure everyone knew it and shared her pain. I still bear the scars.
Now, when my mom first started getting sick a few years ago we started making plans to move her in here with us. She was, understandably, adamantly opposed to it. We decided it would only work if she had her own separate apartment and planned to build her one, figuring that later on after she passed away it could be an office, studio, or house one of our kids that needed some extra space. It was to have its own full bath, kitchenette, etc.
And I suppose I'd provide the built-in maid service. And cook for her because she didn't cook well. And...well, you know. It didn't happen because she got sicker more quickly than we expected. But I sure would've given it a shot. She was worth it. And unlike my Grandma, my husband and my mom did get along. Mostly.