We were on the Epic in December. I think I am in the minority here where I didn't like the thermal spa at all. The first day the entire place smelled like bacon, lol. The staff said that was unusual, and it didn't happen any other day, but it set the scene wrong. Also location is important. Our room was in the front of the ship and it was a huge hike down narrow hallways to get to the back of the ship where the spa was. We only ended up using it twice in our seven days. Maybe if we were closer we would have used it more? I am a HUGE spa person, but the amenities there just didn't feel like the luxury spa I was expecting. For example, no slippers unless you are getting a treatment. Which sounds like a silly thing to complain about, but I just expected the full service.
In reference to access, we had to check in at the spa desk and they took our room card (put it in a binder) and swapped that our for a spa access card. Then when you left, you had to swap back. There were often long lines at the desk helping people book treatments, etc. But if you just go to the side of the desk they will swap it fast for you while helping everyone else.