Personally I wouldn't be comfortable having someone live with us like that. I'd feel like I always had to be on my best "we have company" behavior and wouldn't really be able to relax.
Though it would be nice to be able to run out in the evenings without having to worry about coordinating schedules with the hubby and arranging a sitter for short overlaps where neither of us is available. Plus the time it would save in the mornings, not having to get both kiddos ready and transported to daycare... /sigh.