The Air France lounge in JNB is decent, but it was a bit crowded when I was there and the PCs were erratic.

I had a fairly good experience on DL for ATL-DKR-JNB in B/E, but I fly DL more than CO so perhaps just used to it. Good food (way too much), okay seat. The middle-of-the-night stop in DKR is not pleasant, though... especially since our plane (almost) went mechanical in DKR... fortunately there was a non-rev Delta mechanic on board (sitting in row 1 of B/E) to look at the problem and consult with the operations center in Atlanta... you will never hear me complaining about "too many non-revs" in the business cabin, etc.