I did some playing around with Google Now on a friend's Nexus to sort of see how they compare, and I was left a little concerned.
Cortana's theoretical ability to automatically detect home/work and provide proactive information is great as an idea, but it doesn't seem to work as well as Google Now does.
On both phones I created an appointment with a location and with a reminder for a few hours out.
On the Nexus phone, it showed a card (very similar to how it looks in Cortana) with traffic info and said what time you would need to leave to get their on time. An hour before it prompted a general notification that I had the appointment.
On my phone (Icon) it showed the traffic information when I opened Cortana. An hour prior, as set up, it also notified me that I had an appointment.
On the Nexus, it prompted when it was a few minutes before you would have to leave. Cortana didn't do anything lol.
When the appointment time came both the Nexus and Cortana showed no more information - but the Nexus, recognizing that my friend wasn't at home, showed the time/traffic to get home.
Cortana has not ever shown this, but if you ask will tell you.
What I'd like to see is the ability to always see your travel time to your favorite places when you open Cortana (or be able to select which places you want to see time to) instead of the sort of "mysterious" way it works now.
The same thing with "leave on time" notifications. There seems to be some under the surface mechanism that determines if you get or don't get them.