Theres effectively three levels you can go with these cables.
Basic: £10 ebay lead, free version of VCDS Lite. This will allow you to do the basics of scanning for fault codes and a few other bits, but it will only work on non-canbus cars. Can be a bit tricky to get working due to the way Windows allocates Com ports to the lead.
Basic + Licence: £10 ebay lead plus a $99 licence for VCDS Lite, which unlocks most of the features, but still only works with non-canbus cars. Also pay close attention to the terms of the licence, as its permanently tied to one PC and is not transferable. There is also the benefit that if you choose to upgrade from this level to a genuine cable, Rosstech will give you a credit for $99 you've spent on the licence off the cost of the full kit.
Full Genuine: £250ish for a Genuine lead, which contains the licence built in. Full featured, supported by rosstech, and works on all cars. Can also be sold on in future as the licence is part of the lead/dongle rather than tied to your own computer.
Personally i think that most casual users with a B5 and other older cars will find the Basic option works well enough for what they need it for. Plus, its a very small expense that gets the tool in their hand and they can play with it without having to drop a large sum.