Hi,
I just had a short stop at JNB last month - on our way to Port Elizabeth and back from Capetown - but I found the airport very comfortable - staff was friendly and helpful - many porters would like to help you with your luggage - they hope for tip - no need for that - because there are many baggage trolleys. But we never had the feeling of beeing unsafe. (2 girls...).
All signs are clear and easy to find - connecting flights are in another terminal - just a 5 minute walk away.
Downtown is something different.
Wendy