I've only been to Janissa's Spa at SIBR for a massage, and a mani/pedi on separate occasions. The services themselves are comparable to here in the U.S. I was not overly impressed with the setting though as I was expecting something more with a Wow factor given the status of SIBR. But, it is comparable to a regular spa here at home.
You might also want to look into the spa at the Calabash Hotel. Their spa is set hillside overlooking Prickly Bay, which to me would be a very scenic view. I have not tried their spa services as they were closed last time we were there, but it's one that I would want to try next time we are on the island.