Coupons have finally arrived![1]. They're not perfect but they are stable enough to use in development. If you've got an existing coupon solution for Spree you might want to start playing around with this new one and see if it will work for you.
The main thing missing is the ability to limit to specific products. There is a workaround which involves hard coding the product taxons or some other identifying information into a custom coupon calculator. Shortly after the 0.9.0 release we'll be reach some closure on product groups and we'll be using that solution to enable construction of product criteria for a coupon in the admin interface.
Enjoy!