I wonder how long they keep old threads. There definitely was a long thread a few years back. Basic gist is that the mugs are about 21 oz currently, I believe. They used to be upwards of 23 or 24 oz. If you have one of those mugs, they are quite valuable. But, if you get a 21 oz mug, then you buy a beer for about $9. That depends on the beer, of course. So, that comes out to like to like $.56 per ounce. So, the beer in a 21 oz mug would normally cost about 11.80, but instead is only $9. So, you get nearly $3 in "free" beer with the mug. The mug cost $15 (or used to) so if you drink 6 beers, you come out ahead. On a 7 day cruise, 1 beer per day puts you ahead. Those are rough numbers, but you see where I'm going. Now, most people leave the mug when the cruise is over and take the token (card). Then bring it back next time and you get the same deal. So it adds up depending on how many cruises.
Now, I'll tell you don't buy it planning to use it after the first cruise. Some people report they won't let you use the mug unless you have one charged on that cruise. But, I've never had that happen. I've probably used my mug on 10+ cruises. Depending on how much beer you drink, then it maybe worth a mug every cruise.