Flew JHB-Cairo-JHB about a year ago with Egyptair and also a couple of domestic flights from Cairo to Sharm and Luxor, and cannot complain about the service on Egyptair. At that stage it was 100% better than SAA or Iberia, the only airlines I've used to fly out of South Africa. Joining Star Alliance will probably improve standards even more, so I say go for it.
From operating 2 flights a week to JHB two years ago and now going daily, they must be doing something right.