I read in the Foxycart documentation that coupon codes are proven to improve conversions. However, I think there is some risk involved. I think a lot of buyers may stop in their tracks when they see the field. They see the coupon code field, stop the buying process to go search for a coupon and never come back.
For low priced items, the buyer may just give up searching for the codes and just complete the purchase. However, if it's a big ticket item (>$1,000), the potential discount is large enough that the buyer may abandon the cart looking for the code.
Thoughts? Experience? Any research that I should read?