When Windows Phone 8.1 was first released, the most anticipated new feature was Cortana, Master Chief's personal assistant. Microsoft went all-out on the implementation of Cortana for Windows Phone, going so far as to bring the voice actor Jen Taylor to voice the predictable parts of the character. As time went on and Microsoft added new features, such as jokes and impressions, Jen continued to voice those answers.
As for usability features, Microsoft had some really great concepts included into Cortana, such as known addresses and natural language processing, making access to these features more natural. Saying "Hey, Cortana. Get me to work." could easily open your primary navigation system with the address for your office pre-entered. Cortana was able to determine those known addresses on her own by seeing where you go often and offering to name them for you. Those features later came to the other platforms care of their own assistants.
In Windows 10, Microsoft has really increased the amount of information that Cortana keeps in her notebook. With events and meetings to packages and flight information, Cortana is getting to the point where she can keep track of just about everything for you. That's great when you're at home or on your tablet, or if you are already running Windows 10 on your phone, but what if you have Android or iOS?
A beta version of Cortana for Android was released several months ago, obviously with some features missing. Google's operating system is not as developer-centric as Microsoft's (they have been building developer tools for over 3 decades), so Cortana does not have access to data contracts like she does on Windows, meaning she can't gather quite the same information to help as she normally can, but she does provide a lot of help for Android users anyway.
This week, Microsoft has launched its private beta of Cortana for iOS. Like with Android, Cortana is not full-featured because of Apple's massive lack of developer capabilities. On Android, in some circumstances, Cortana can be set as the primary assistant, but on iOS, Siri will always be the not-quite-sassy queen. But, if you launch Cortana, she can still make your life a lot easier.
The only issue right now is that the way Apple distributes private software is through a platform they purchased called TestFlight. TestFlight has a limit of 2000 devices that can be published to, meaning that, for the time being, only 2000 users are getting access to this software. Hopefully this particular test will be successful and quick, giving access to the rest of Apple users in the near term.