The 789's being delivered to UA have a special engine performance package on them that gives them the legs to handle both passengers and freight on the EWR-JNB route. Ref this post in the Polaris Seat refit thread on the UA forum. It means you will get UA's best business class seat (if you book business class).

I am seeing economy fares around $950 and business class fares around $3100 for ORD-EWR-JNB round trip from the time UA starts flying to JNB through the end of August. I was seeing total flight times of around 21 hours in all, including longer (3+ hours) connection times.