I think the mug cost $10. It holds a bit more beer than a standard draft (22oz vs. 16oz, I think) but costs the same to re-fill. Yes, it's good in any of the bars on the ship. You can also have it filled with soda for the price of a regular soda at any of the bars.
I once did the math and I think that if you're drinking Heineken, you need to drink at least 8 beers during the cruise to make this
pay for itself. 10 if you're drinking domestic beer. While it's worth the price and makes a nice souvenier, I found it kind of a pain to drag the thing around with me (it's fairly heavy), so I did not get it on any of my subsequent cruises.
A word of warning: the mug does not fit in the drink holders in the Walt Disney Theatre, so if you're going to enjoy a beer with the show, leave it in your room, or you'll have to hold it the whole time. Better yet, go for one of the Pub Draft Guiness cans they serve in the theatre.....