I have a BB Torch on Prepaid from Vodacom. I also have a contract with MTN. The plan was to get BIS on my MTN contract so that I only had to carry one phone (my BB) around but when I enquired about BIS and found out that it would cost me more on MTN contract than any other provider or on prepaid, I ditched the plan.
I'm waiting for my MTN contract to expire and then I might port the number to CellC Prepaid, or just cancel it entirely and use the Vodacom prepaid SIM.
It's the principle of the matter - MTN is charging between R10-R12 more for exactly the same service, breaking the "R60 BIS" RIM-prescribed "rule" for Blackberry pricing.
As to BIS speeds - I get 3G and EDGE speeds depending on where I am. I download apps and updates, chat online through my GTalk, browse a horde of sites and send a million BBM messages to friends and colleagues. For me the big saving comes from BBM and GTalk (Nimbuzz) - I barely send SMS's anymore. The equivalent number of messages sent via SMS would be well over R60 in airtime or bundles. The "free" internet is a very nice convenience. I don't have to worry about how much data my apps are using in their updates or if I'm going to hit a limit when I download stuff or send my friends pictures and such.