Drink packages can be used on the ship as well as Coco Cay and Labadee. Drink packages do not cover anything ordered from room service, not even cans of soda.
A good deal depends on the person. Generally the rule of thumb is that you break even around 5-6 drinks average per day. This is assuming you don't get it on sale and only use it for alcoholic drinks. Fresh squeezed juices at breakfast, specialty coffees, bottled water, these all normally cost money but are included in the package. It also depends on how long your cruise is IMO. Some people experience drink fatigue and after a few days they find themselves not wanting to drink more. A port intensive itinerary also might reduce the value as you might be off the ship quite a bit. So really it is a very personal decision. For us, on a 5-day with 2 sea days and a visit to Coco Cay it makes sense and we could easily get our moneys worth without ever being more than slightly tipsy. A 9-day with several stops we would be much less likely to get it.