The mug was great for me... I bought it right after boarding and started using it right away. The token (card) was really nice, not having to carry around the mug all over the ship.
A normal beer is supposedly 16 ounces, but is more like 14-15. The mug is supposed to be 22 oz, but the pour is more like 20 usually. You get the mug filled up at the 16 oz beer price. So the match works out to be about 4 beers for the price of 3.
4 15 oz beers = 60 oz
3 20 oz beers = 60 oz
So figuring $5/beer, which is on the low end on the ship, you'll pay for the mug after 9 beers or so. After that it's all savings, and you get to keep the mug.
I'm a 5-7 beers a day kinda guy when I travel, so it was a no-brainer for me.