For me, the location based reminder is beyond awesome. I have used it several times since getting Cortana and it is incredibly helpful to me. I am also so pleased to be able to add calendar events via voice and have used that too. I am hoping we will get voice-add for email as well in the next round of updates.
I also really like being able to say "what is this song?" rather than scramble to hit the right buttons to get a song identified. Hoping we can voice-start podcasts next round though, I play podcast much more than listen to xbox music, so voice activation on that would be great.
Oh! Also setting quick one time alarms, such as "Wake me in 30 minutes." That was nice to do yesterday.