I haven't used Microsoft Cortana, so I'm not sure if it's any good, but they reportedly get one thing Google completely misses: people want a personal assistant, not a creepy stalker. A personal assistant takes care of the kinds of tasks you tell him or her to handle; a stalker does things he or she thinks you want. Big difference. My personal assistant would schedule meetings at work. My stalker would send my mistress flowers for her birthday.