Beverage plans are priced based on a daily rate so if the daily rate is $42 per person (and this varies too based on what you drink) and you plan on having a couple of glasses of wine a night, it might not be the best value for you. And the price is based on the length of your cruise, so you can't purchase a 5 day plan for a 7 day cruise. You're all in. If you plan on a couple of umbrella drinks while sitting at the pool, a cocktail while you're in the casino and wine with dinner, it may be right for you.
There are beverage plans for soft drinks and juices, which could be great idea if you're traveling with children, beer and wine plans, and Premium Drink plans. You can purchase these prior to sailing or when you board.