I did the SAA jfk to jnb flight in business class in November.
The plane and layout wasnt the best. It's an older version and the seat is a bit dated. In full lay flight mode, the main critique was the seat felt like three separate chairs being pushed together. It was adequate but not the best. 2-2-2 layout with unlimited leg space. Very spacious cabin but lacks privacy. Food was adequate, service was standard.
The reason we chose that route was because of the non stop option. We saved 8-12 hours by not having to stop in Europe. The SAA flight from IAD sometimes uses a newer plane, so it is worth looking into if you need a positioning flight anyway. Delta also has flight from ATL direct to JNB.
Another con was the time of departure on the outbound. 10:40am made it difficult to fall asleep. Also, the contracted Swiss lounge at JFK is a joke. However, the inbound departure time of 9:30pm was ideal. It was extremely easy to fall asleep especially after a week on safari. JNB lounge was crowded but big enough to support the crowds. Above average food offerings and bar service.
Keep in mind, if you book through United, you are allowed one free flight within Africa as part of the excursion perk. We did JFK-JNB-CPT on outbound, CPT-JNB a week later, then JNB-JFK home. Total amount was 160,000 miles per person, all in business.