Can I pull a report of all my customers that have used a discount code?

You can do this easily by creating a custom roster from your dashboard > Roster & Analytics > Rosters (Refund, Attendance, Add Students) > click Custom Rosters toward the top of the page

 

Click the pink "+Create a New Custom Roster" button, select your filters (date, location, or programs), select some sessions or 'all matching sessions', then click 'Next'. Proceed to select the fields you like to see in your report, making sure to include the fields 'Discount code' and 'Discount code amount', and order the selected fields as desired. Name your roster and save and download it.