Ok, so I got the freestyle mug with the dining plan this week. It really depends on how much soda/Pop you'll drink during the day and if you're willing to stand in line to get it to determine if it is worth it.
Unlimited refills that day in the parks including both Coke-a-cola brand beverages and icee frozen drinks. The average cost of soda is $3.00/cup and icees are $5.00, so if you fill it 5 times with soda or 3 times with icees you will have equaled what you would buying these items out of pocket.
Expensive - they cost $14.99 each of purchased individually or as part of the quick service dining plan.
You can only get a refill every 10 minutes and water counts against that (I used a small amount of water to rinse my cup at a freestyle machine and then wasn't able to fill it for 10 minutes).
You have to carry the mug with you and then have to get a locker to store it on many attractions.
You can only use it in the parks, not at the hotels.
So I would say it's worth it if you're already having to get a locker, don't mind waiting in line for drinks throughout the day and will drink enough to resolve it a minimum of 3-5 times depending on what your drinking.