For what it's worth.
During my two weeks this Christmas, 12/21-1/4 with most of the vacation at BWI and the last day at POFQ we had no problems with our mugs when used for counter service at resorts or otherwise. Not once did we run into any soda fountain that was not reading and requiring the RFID tags at BWI, YC/BC, POFQ, Poly, or CR. In fact, POFQ was the 15th day and our mugs had to be re-activated for the last day but were activated with no issues.
My observations:
Several guests during our stay tried using the machines with "old" mugs and simply shrugged and left. A few moved to tea, cocoa, or coffee and grumbled especially a family of seven with young children who were quite irritated that they were just there last year and had not budged for new mugs. I did see a few groups walk up and purchase additional mugs for their children after trying their old mugs, I would bet they are the primary target of the program.
Most guests had no issues with placing their mugs on the readers and pressing the fill button, only once or twice did we see a guest have any issues and they were using invalid mugs anyway. At BWI, the cashier explained the fill process immediately after we picked them up which I would guess eliminates many of the issues some have reported on this thread.
I am like another poster or two and drink a very high volume of beverages, Diet Coke, Coke Zero, Light Lemonade, coffee and hot cocoa primarily with the mug. Only once or twice during the trip did I get denied a refill and had to wait a minute or so longer than I wanted to which could be irritating but nothing worth getting worked up about. I can easily drink a half gallon of water in a few minutes when hot or very thirsty so I am probably a more extreme case for refills.
I continued to see guests using all manner of cup, mug, thermal containers to fill up on tea, coffee and hot cocoa at the Bakery at the BWI and at no time did any CM say a word to them and I can't blame them.
Personally, bringing an old mug back shows intent to steal beverages knowing there is a cost associated with the new ones. Disney is an expensive place to visit and most people look for any opprotunity to save a little but reusing old length of stay mugs is stealing in my mind. If someone is luck enough to have purchased a lifetime mug, Disney should honor their commitment but I have only seen guests using the mugs from the past few years.
I complely understand wanting to save the $72.00 it costs for four mugs but a quick trip to Wal-Mart, Hess, Target to pick up sodas and put them in the fridge would likely save nearly as much and you don't have to make a special trip to the CS location in the resort. In addition to our purchased mugs, we saved our Coke Rewards points over the year and redeemed them for free 20oz Coca-Cola products, over 70 in all and redeemed them at an area Wal-Mart which cost me nothing except the time and gasoline.