I find Cortana the fastest way to set reminders and calendar appointments on my phone (not so much on PC). I can press the search button to start Cortana and just say "remind me to do "blah" at 7pm" and she puts it straight in. No finding the calendar, opening the day/time then typing in a box, it's VERY quick. Also I like it for location based reminders too, eg remind me to get bread when I'm in ASDA (UK supermarket), it works very well, again on the phone, less useful on the PC or Surface then. Also "set alarm for 6pm" is a lot quicker than finding the alarm app, setting the time etc.
ON the computer or surface I don't really use voice search at all, but I do occasionally open the Cortana app to see the relevant things she's showing me that I might be interested in.