It's also country and regional market dependant. My country the Netherlands doesnt naturally get Cortana. It's part of the west-european market, I have not see or touched Cortana first came online. We even didnt have Tell Me back in windows 7. And yes it apparantly is also very device dependant.
I think and even more relevant issue with Cortana is that they only support 7 languages now, maybe up to 25 later, but it's not enough. When windows 10 is available for all mobile devices that can support Cortana, I hope that microsoft will also have speechlanguage packs ready for all languages. Otherwise Cortana will be quite a handicaped device, because of recogntion errors in foreign names, geographical locations and local custosm in humor and expressions. The English laguage pack can not cover all of that, and many also can not speak perfect english. If microsoft also wants an affrodable windows phone device in developing nations, they still have a lot of work to do with Cortana. I do not think that Cortana will be a priveleged service for everyone as they would like us to believe.