I've never hired a service in over a couple dozen trips. The SAPS 520 Form is long but, you only need to fill out a few certain sections. I do it so much, I have a SAPS 520 scanned in and set up in a form filler program with just those sections. Type the info in for myself and anyone traveling with me. Takes 15 min.
Print a copy and make sure you have a copy of your invitation/motivation letter, US Customs Form 4457, airline ticket(s) and passport - the same stuff you have to supply to a pre-approval service. I print out and make three sets. One put inside the rifle case (just in case the rifle misses a flt) and carry two in my carry-on. There are a couple of advantages of using a service.
They have your completed SAPS 520 already done/approved when you land and those are usually the first ones cleared at the desk. The service may include meeting you either at your arrival gate or just inside the main reception hall and walk you the couple hundred meters or less to the SAPS Firearms Control Office, clear your rifles and either take you to meet your ground transport if you're staying outside the terminal complex or walk you to either the City Lodge or Protea hotels within the terminal complex. I don't know if they collect you up the next morning to help you check in on any follow-on connecting fts.
If you're going during the high/peak months, June, July, Aug. There can be a 30-45 min queue at the SAPS office. Outside of those months, maybe, 10-15 min. Mostly waiting for the rifle cases to be transferred from the aircraft to the office.
For a first-timer, it might be money well spent mostly for the piece of mind but, once you've been thru it once it's actually pretty easy.