My DH, DD7 and I stayed at Pop during the long July 4 weekend and bought one mug to share between us. To be honest, I don't think I got my money's worth because I brought milk, juice, soda, beer and bottled water along with a cooler and tons of snacks, cereals, fruit, bread, cold cuts, etc. I used it for my morning coffee and for DD7's lemonade. Over the 3 days we stayed there, it didn't pay off. But, for longer stays, I think it's worth it, depending on your drinking habits... meaning, how often are you going to go to the food court to refill it? how long is your stay? At about $2 per drink, it's just a matter of doing the math... if you'll use it 6 or more times, then buy it; if not, then forget it.
As far as food brought into the park, I asked CM's if it was ok and no one has told me you can't do it and I've never been stopped at the gate for bringing it. I always pack a backpack with bottled water, juice and snacks. Sometimes I have brought sandwiches.
When we go to the water parks, we bring a collapsible cooler and we bring fresh fruit (which tastes so good after you've been in the water...), bread and cold cuts, plus other snacks, juices, sodas and snacks and we prepare our own lunch. You can buy a mug at the water parks for $10 and use it the whole day, but it's only good for that one day. I believe they don't have any restrictions on bringing food to the water parks, except they don't allow you to bring alcohol.
One last note.... I DO enjoy drinking my morning coffee out of my POP mug.... It reminds me of great times, so yes, for that alone it was worth it!