That depends...I mean, you'd have a second opinion on everything you do, and you'd have an extremely advanced mind giving you advice and figuring out problems faster than you can get to them. On the other hand, you'd never be truly alone. Even when you just need time to yourself to think, you'd have somebody looking over your shoulder at whatever you're doing. (Yes, even while you masturbate.) I'd probably want a less advanced one, one that can figure out the answers to advanced problems and maybe one I could converse with. And no, it's even stated in FoR, Cortana cannot control one's body or mind, though she can amplify certain physical responses like reaction time.