Grand Celebration is a great bargain. While the ship is old, it is well maintained. The staff is just tremendous. In fact, I have cruised a number of times (over 30) and have never met a happier, more outgoing crew. Everyone was well trained and good to deal with. In addition, the entertainment is actually first rate. I tend to avoid the cruise line variety shows and went to theirs just to check it out. It stunned me that it was so good.
Downsides: Do not go to the upscale restaurant. While the staff is great, the food is quite average. I would not spend the extra money for the dinner. The other restaurants are average cruise ship standard. However, the pool hamburger bar does not provide hot buns for the burgers, which is a personal pet peeve of mine.
Upsides: Buy the drink coupons. You can't beat the deal provided by those. It is about a 35% savings from "rack" rate. You can purchase these at any time. We bought ours walking onto the ship and never regretted that decision.
The only downside was the fact that room air conditioning did not chill the room as much as I would have liked. However, the staff immediately provided a circulating fan so we would stay cool. No stress, just brought us a nice fan.