They are awesome dogs if you train them well from the get go. Not generally a good dog for the inexperienced. They are hounds and almost impossible to train to retreive, so if you like a game of fetch, look elsewhere.
They get big. South African RRs (working stock) are a lot larger than AKC spec. My RR (unfortunately no longer with our family) was over 100lbs at 6 mo, and topped out around 145-150lbs. AKC spec is around 90lbs for a dog and slightly less for a bitch. In any case, all muscle.
They have fantastic temperments and are great with kids, but not great around really young kids because even the pups will knock kids down in their excitement even with the tail wagging.
They tend (lots of exceptions, but...) to like other RRs and not be too fond of other dogs. Early socialisation is a must.
They are lazy and smart - meaning they are voracious counter surfers and will eat anything not encased in concrete. They will figure out how to open your gate, or do anything else they really want to do. They love to run, almost as much as they like to just lay at your feet.
They are very attached to their owners, like most dogs, and really, really do not like to be alone. Not a good "outside" only dog, unless you are outside most of the time.
They are awesome dogs though, and I loved mine. Check out the rrcus.org, regional clubs, and ridgeback rescue sites for more info.