I have good news all: I have in my hands my very own US B1/B2 visa, valid for 10 years (my very first US visa)!
Thank you to everybody who gave me such useful advice here, I referred back to it many times.
I also wanted to share my own experience in case it helps anybody (and there are some facts here which will be a big help for others):
1: If your appointment is at the US consulate in Johannesburg, take a hat and sunscreen. I had to stand in the baking sun for an hour with about 50 other people ... the lucky 10 get to sit in the only real shade on a little bench, but otherwise you will be told to stand in queues based on the time of your appointment. It doesn't really help to rock up early to the appointment for that reason, unless you want to work on your tan. If you have children or old people with you, maybe bring a folding chair? Oh, and park in Sandton City across the way, there is no parking at the consulate (and no drop-offs allowed outside it).
2: EXPEDITING VIA DHL: This is like some amazing hidden secret. I say 'hidden' because I didn't see any mention of it, but apparently I was meant to have received a piece of paper with the fact of this service on it (the DHL lady said so anyway when I asked her about it). Basically, inside the US consulate there is a little DHL booth in the corner. After your interview is done, if you are told that your visa is approved and you urgently need to get it expedited because of a pending trip, DO walk to this little booth afterwards and ask for this service. Be prepared to pay R268 for it (they accept card payments). The lady told me my passport would be ready for collection in 2-3 days from their branch in Rivonia (only option for this expediting), while the standard time (as per the US agent who interviewed me) is 3-5 working days. In reality, I got my sms from DHL at 13:00 today (Tuesday) after only having my interview at the consulate yesterday (Wednesday) at 14:30. That's less than a day turnaround from interview to collection at DHL (I went directly to the Rivonia branch and my visa was ready) -> that's the fastest I've heard about it, and thank goodness too because I'm flying out this Sunday! Bottom line, if time is of the essence, go pay that R268. DHL's sms arrived while my visa application was still showing as under 'Administrative Processing' on the US visa website (it's finally changed to Issued now).
3: Don't freak out by 'Administative Processing' as the status on your visa, if you were told that it was approved. I almost got a heart attack when I saw that online (Administrative Processing usually requires weeks of delay because they're investigating something) but apparently this is perfectly normal and it's also the same term used for just the creation of your visa after a successful appointment. You will be informed at your interview if the application was successful or sent for the real administrative processing (as somebody ahead of me in the queue was informed) or rejected outright (another poor guy got that message).
All in all I feel very lucky!!! I hope the above helps somebody, and if anybody has questions just let me know
