First, since Crystal Serenity goes to dry dock next month (October 2018), some of the comments may not be applicable.
My wife and I took a Crystal river cruise last year (Danube) and were so impressed with the level of service and the shore excursions that we booked the Italian/Greece cruise on Serenity. While we enjoyed the cruise, the level of service did not even compare to that of the river cruise. In talking to long-time Crystal cruisers (some over 10 trips), they say the quality has gone down since Crystal was sold and the bean counters took over. That is our opinion also—with just a few differences, we saw little to distinguish Crystal above other cruise lines.
Pre-trip support was not that great. It was like pulling teeth to get transfer information (bot pre and post cruise). The only option I got was IF I booked their hotel, with pricing about 2 to 3 times what you can get online. I was given no transfer options post-cruise, yet found out once onboard they do offer transportation from the port (Civitacecchia) to the airport, but we had already arranged our own since we were not informed of that option beforehand. It does not show up in the list of excursions at the beginning or end of the cruise. This is completely different than the river cruise where you could easily select transportation before and after the cruise.
Cabin was very spacious. The carpet was very worn, but still acceptable (no holes, nothing to trip over, just very faded). Plenty of closet space.