In most cases, the coverage map is a bit on the conservative side to prevent giving people false hope. This is to prevent giving the sceptics another stick with which to beat Vodacom for false advertising or ripping the customers off, etc.
However, there are still cases where the map may indicate coverage but there is no coverage. The coverage predictions are done on a planning tool which takes into account the general terrain and clutter (trees, buildings, etc.) but this geographical information is not always 100% due to erection of new buildings, trees which have grown, etc. It is merely meant as an indication and it is still preferable to check the signal at one's residence before buying any 3G contract. This can normally be done using a 3G phone or a borrowed datacard, etc. and should give a indication of what sort of performance you can expect.