I don't have personal experience with HSS but I read medical journals for a living and the doctors at HSS do a great deal of research and are known for developing many surgical techniques as well as prosthesis and rehab protocols. They also only handle orthopedics and rheumatology so everyone who works there have a great deal of experience in the field as that is all they do. They have consistently been rated #1 or at least in the top 5 in the US News and World Report listing of the best hospitals in the US for musculoskeletal problems and surgery.
I have a friend who has a bone disease that had numerous surgeries at HSS through childhood up to adulthood and she said they were the best. She would even fly from Los Angeles back to New York if she needed surgery.
They also do the largest number of TKA and THA per year in the US.
If you can get some referrals to a specific doctor you will probably have a very good outcome.
Though even the best hospitals can have a bad day, overall they do have an outstanding reputation and are clearly at the cutting edge of orthopedic surgery.
Simon