My partner and I just went on our third Atlantis cruise in Jan, and are doing our fourth in July (Baltic!).
Atlantis cruises are whatever you want them to be - if you want 7 days of circuit parties, it's there. If you want 7 days of relaxation - spas, pool, great food, entertainment, it's there.
I think this sums it up - on our first cruise in 2003, we boarded the ship, unsure of what we had just gotten ourselves into. We made our way to the pool deck and a cruise line staff member started chatting with us (she was probably in her mid-20s). She had been working for Norwegian for a short time. She said, "No, you guys don't understand - we (the crew) have been waiting for you guys ALL YEAR!!!"
For 50-51 weeks a year, the crew on the ship is treated to kids, families, and old folks. For 1-2 weeks a year, they party along with us.
Now that we've been on a few Atlantis cruises, I describe it to friends as summer camp. There's the excitement at the beginning, the fun and new experiences during, and the genuine sadness at the end (although hopefully made easier by 'See you next cruise/year!')