On my third cruise I was able to book a cabana on Serenity Bay. I think it was a total fluke because it was my 3rd cruise and I booked it super late on the day it was open for everyone to book and it should have been booked up by then. Anyway, I booked it with the thought that it was too expensive and I'd probably cancel. I didn't end up canceling. The day I booked it was my 40th birthday and I decided it was a treat-myself moment. The was great to do it once, we enjoyed the comfort of the cabana furniture (and fan) and the service we received. I had one of the greatest naps in that cabana, the sounds of the waves and a nice breeze was great.
What we didn't like though was there was no bathroom in the cabana and no shade over our hammock until the last hour we were there. I wouldn't rush to book it again but if I was with a larger group it might be worth it cost wise.