I was on Sensation with my teenage son July 29/Aug 3, 2007 and we did the glass bottom boat in Freeport (make sure to take dramamine - big waves out where they take you!) and the Atlantis tour in Nassau. Also booked the Historical Highlights Tour but couldn't manage facing the heat and humidity again - I plan to do that bus tour when I'm back in Nassau on my June 2009 Valor crusie.
If you want to really use the Atlantis water park and all that, and plan to spend most of the day there then it might be worth the cost. Their aquarium is nice - best part for us were the huge ray , Zeus, who I believe has been released into the wild since then because he was so big. I have video footage of him. The resort is gorgeous and I can imagine relaxing there, we just did the tour of the Dig though,which includes a very brief walk through the interior of the resort/lobby.
Check out my several pages of Sensation photos at the link below and ask me any questions.