Depends on how much you drink, and how long (and how many port days there are) your cruise is. For me 3-4 cruises I think it is worth it FOR ME, however anything longer than that it isn't worth it FOR ME.
An other thing to think about is specialty coffee, milkshakes, soda, mocktails, bottled water, Gatorade type drinks, and energy drinks are included with CHEERS! (but don't count towards your limit of 15).