We sailed on the Celebrity Xpedition in May 2018 to the Galapagos, on the inner loop itinerary. As many others have said, the service, food, and crew were excellent. We were met at the Quito airport by Celebrity and basically were in there hands until we returned to the JW Marriott in Quito a week after boarding the ship. All aspects of the trip were well done, and every one of the Celebrity folks was helpful, and easy to work with. The naturalists were knowledgeable, interesting, and really enhanced each excursion. Also as others have said, the excursions are why you sign up for this cruise.
Others on this board really helped me decide what to pack and what to leave home. I strongly suggest you get a list of items to pack and follow it - especially the wide brim hats, shoes, long sleeve shirts (for sun), and sunblock (at least SPF 50). My wife and I are 66 and 67 respectively and in reasonably good shape with minimal problems getting around. That said, we were pretty tired by the end of the week. As explained in other posts, the daily itinerary follows the same pattern - a morning excursion, back to the boat by lunch, and an afternoon excursion. Almost all of the excursions have an easier and more challenging flavor. We did the more challenging on all but a couple, which involved long, climbing walks... The snorkeling also comes in two flavors - "off the beach", which is exactly as it sounds, and deep water - which is where you are taken by zodiac to the snorkeling spot and jump in off the zodiac. Water is too deep to stand, and currents can be much stronger. Water temperature was about 74 - 77, and Celebrity provides wetsuit, mask, snorkel and fins.
One suggestion - bring some sort of book/app/device that will help you identify constellations. Star gazing is outstanding - no light pollution.
We had a chance to upgrade a week before the cruise, took it, and were glad we did. Room was clean, service excellent, and the veranda was great for star gazing...