Thanks a lot for the feedback. My comments:

1. I agree, main problem would be to get users to keep using the app in the long run.

2. Yes I started researching now and think that it's probably too crowded and not enough way to differentiate yourself

3. I was thinking along the line of promo codes. Send a code in the email, and keep it in the database until the user redeems it. Then over time we could add mobile apps and play with geolocation and beacons.