This was the first cruise for us. We selected RCI on the advice of a friend who is a seasoned cruiser and we weren't disappointed.
Embarkation was a bit confusing at first as we had never done this before. Once I realized it was pretty much like a more crowded airport it became easier.
We fell in love with the ship as soon as we boarded. After a few days I could see where some people were correct when they say Mariner is starting to show her age, but it didn't detract from the experience for us. A little bit of paint and some new carpet in the halls and she'll be as good as new.
A bit smaller than I expected as I've never cruised before. Pictures on cruise line websites are obviously taken from angles to make the rooms look bigger. We got used to it pretty quickly and it wasn't an issue. Plenty of closet and drawer space. Very quiet part of the ship most of the time. Close to the aft elevators.