Fred,
You don't need to pay the $3 extra for a mug and lug it around. The sticker itself is $35 for a 7 day cruise, $20 for a 4 day, and $15 for a 3 day. Showing your KTTW card with the sticker affixed is all you need to do to get a soda. They will give you a cup (plastic or glass depending on venue).
We found the soda card particularly useful for DSD to get a soda to have during the shows. Saved a lot over buying smoothies from the vendors!
You need to buy a little over three sodas a day to make the card worthwhile (keeping in mind that soda is free with meals at the sit-down restaurants).