Microsoft's Cortana has been available on Android for about 2 months, but shortly after its launch, it got an update to remove the Hey Cortana hotword (though that remained functional in Cyanogen OS) because of potential conflicts with Google's own Ok Google. That dampened a bit the app's utility as it became slightly more difficult to launch when in a hurry.
This update to version 1.4.0 tries to circumvent the issue by adding a homescreen widget. It has a big Ask Cortana button that launches voice searches immediately, a list of your upcoming events and reminders, and another button to add a reminder. It should let you get to most of Cortana's functionality from the homescreen. The update also added some needed load time and stability improvements.
The new version 1.4.0 includes the improvements:
1. A new widget. For a quick launching Cortana and set reminders.
2. Reduced app load time.
3. A better loading experience for low speed and unstable network conditions.
4. More stable voice interactions.
Cortana is free but appears to still be limited to the US and China — this doesn't seem to have changed since its launch. The app and its new widget are only a few taps away at the link below.