I have done some thorough testing of Voip products - specifically Softphones.
The quality on unshaped is superior to shaped, BUT shaped works fine after hours. There was a definite improvement during the peak hours. I must qualify this by saying if you have a lot of other traffic (like email, browsing & FTP) going on at the same time as when you want to make Voip calls then it is not worth the switch. I would recommend switching if it is to be used for dedicated Voip calls, with a little of the 'other' stuff.
I would not recommend going the 12 or 24 month contract route should you switch to unshaped adsl, because I suspect before years end the Gov. Will force Telkom to offer unshaped, if the launch of SNO products does not.