The lounge in JNB is poor, so I wouldn't recommend spending 5 or 6 hours there. If you did decide to go into Johnanesburg, you could easily take the Gautrain, which would have you in Sandton in 15 minutes. Now, Sandton is not exactly the 'real' Johannesburg, but it's certainly a lot nicer than being stuck in the airport! You could have a drink in one of the restaurants in Mandella Sq and watch the world or take a wander round the shops. Maybe even have a massage - 90 minute full body for £30-£40. Great value with such a weak rand! As much as I would love to recommend a trip to one of the local lion parks or other tourist attractions, I would advise against taking a taxi anywhere outside of Sandton, as the traffic in the area can be horrendous and you would seriously risk missing your flight, so the Gautrain is definitely the best option.