the 737 j is really not that bad. i flew on it the other month and it's fine for a 2 hour flight. catering was great. no ife. you're going to sleep on the jnbcpt leg anyways. it's just 2 hours.
the domestic sa lounge at jnb is alright. flashy architecture and decent catering but really nothing too remarkable. it was am and all they had was salad, cold cuts, cheese, crackers. i tried taking a shower and there in january was no water pressure. i wouldn't deliberately connect in jnb longer for it. sa's lounges are really unremarkable, but by no means ua clubs in the usa bad. maybe i'm spoiled because i hang out in the ua and sq lounges in lhr.
also bear in mind when meals are served on domestic flights:
http://www.flyertalk.com/forum/south...s-cpt-jnb.html
and i've heard they won't serve booze on flights until after noon? or 1100? look around the forum
keep your expectations low and you'll have a great flight on sa