I would also suggest waiting until you board to decide. Depending on the sailing, the lines for specialty coffee at Cafe Promenade can make it not worth it. I have gotten it for a couple different cruises, and decided not to get it anymore after having to wait almost 30 minutes in line for a cappucino.
This also makes it hard to actually get your moneys worth since you may decide it isn’t worth your time standing in line and only use it for one or two coffees.
As an fyi, I have also ordered cappucino at the bar when you walk into the Windjammer. But it is the same story of waiting because there usually is only one or two people working the counter there.