Neither my husband or I, or my parents (in their 70s, with some limited mobility) had ever cruised before. It was a wonderful trip! We sailed out of Miami (my parents stayed at the Embassy Suites near the airport prior to sailing, and got the "cruise package" with transfers to the port). We flew in the same day and got an easy cab ($24 from the airport to the port).
We spent the extra money for the concierge level and would do it again. For the extra $150 per cabin, the complementary champagne (and maybe other extras, I don't know if everyone got fruits and snacks and their water pitcher refilled during the day, turn-down service with chocolates, etc) and the balcony view was well worth it. Treat yourself if you can! Literally, my favorite part of the trip was our cabin stewards asking "Did you have a nice day, Madam?" It made me feel like royalty!
I didn't take advantage of any of the spa services, but for no charge, I loved the fitness facility (Take a set of headphones with you to use on the cardio machines) and the AquaSpa cafeteria (open for breakfast and lunch with almost no waiting and wonderful health choices!).
The food, the food, the food! Eating in the grand resaurant for dinner is really a chance to eat a 5-star + meal every night for five nights. The waitstaff is top notch!
We went to each and every show in the Celebrity theater and loved it! Singing, dancing, comedy. You know that these folks really put their hearts into putting on a good performance; it shows!
Never had to wait for a deck chair, and could go around the pool, or even to close to the stern for a great view and lots of quiet.
Lady luck wasn't with us in the casino, but the staff was professional.
We did not book our shore excursions through the cruise line, but if my non-internet savvy parents had been traveling by themselves, I would have insisted that they do. You probably could not go wrong with any of the choices.
Would recommend taking advantage of the opportunity to dress with a style. Active wear is fine during the day (t-shirts and shorts), but after dinner, plan to wear dress/skirts and slacks/jackets (not necessarily with a tie, except on formal night).
I don't know if we will be "cruisers," but if we went on one again, it would be with Celebrity.