biggest concern will be that VWoA does NOT warranty B20, they only warranty up to B5. there are rumors that might change, but as of now if you have a fuel system problem and they find out you were using >B5 you'll be stuck with the bill yourself.
Shell is a reputable dealer, but it would still be good to ask about ASTM certification for their fuel. the biggest problem with BioD (as it is with regular D2) is BAD fuel. there is nothing inherantly wrong with BioD, but if you use someone's homebrew and they haven't properly made the fuel you could end up with all kindsa weird stuff in there.
i'd recommend you try out the B20 if you're willing to take the "risk" on your warranty. it's better for both the environment, and your conscience.