If you had done the maths you would notice that Pre-Paid Vodacom (and MTN) works out cheaper than ADSL for the for people who use up to 3GB a month. Here is a
comparison I did for VGS against ADSL but in the last column you can see how 3G stacks up as well.
Only if you are generating less than 250MB of traffic a month does Virgin OOB rates make any sense (if you can live with EDGE speeds).
I've been purchasing 2GB bundles (effective in bundle rate of 19c) and manage my usage so it lasts me 30 days. Yes roll-over would be great, but when was the last time Telkom let you roll over your unused ADSL cap? Besides that is the great thing about pre-paid, if you know you are going to using less data this month then buy a smaller bundle.