Yes, the ESPN mugs are glass. They also had a Disney Cruise line mug that had the same privileges. My wife and I got one of each for souvenirs (and yes, they paid for themselves!).
As Tiptoen said, the ESPN mugs (and their Disney Cruise Line bretheren) allow you to purchase a 22 or 24 ounce beer for the price of the regular draft beer (16 ounces?). You buy the mug for $10.00 and then reuse it the rest of the cruise. It's a bit more cumbersome because you have to lug it around, but since I'm a beer drinker, it paid for itself by offering more beer for the buck.
If you're on deck, be sure to head over to the adults only section and talk to Nicholas at the Signals bar... he'll give you a chilled mug when he refills it for you.
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