THere aren't many places that offer it - MSKCC is one. It is usually done by those interested in academics as a career. It wouldn't be incredibly different from a general surg path fellowship though, depending on how it is structured.
Depends on what you want to do. Do you want to work in a cancer center forever?? If so, awesome by all means go to Sloan Kettering! Oh, you want to work in small community practice?? Really? Then it is definitely not worth a year, trust me I struggled over this same thing some years ago when I had the option of going to MDA and MSK. I chose wisely and did a board cert subspec.
Onc path fellowships train you to sign out THEIR cases, butt rare tumors sent in consultation, difficult operative candidates usu. bone/soft tissue oriented, lots of neuro, lots of wierd crap in essence.