There's absolutely no reason NOT to join any program once you cruise the line for the first time, unless you utterly hated the cruise and resolved never to go with that line again. Celebrity's small charge is worth it if you ever expect to cruise with them again.
The perks are generally delivered as advertised, although many of us believe that the typical "one category upgrades" carry so many restrictions that they are seldom worth a hill of beans--especially if you're working with a good agent who is getting you a big discount to begin with. But what the heck, you never know when a hundred dollar coupon may show up in a mailing that fits in with your plans.
You can join most of them for the cost of a stamp or a few keystrokes, so why not?