I won't suggest for teleconvertors / extenders as Khurram said its a huge light loss plus in almost all convertors .... there is one common problem.
They make the image quite soft except the centre. Only area in the middle of the pic will be acceptable and corners and towards edges it would feel like depth of field effect even if shot at f9 or f10. I bought one for moon shots and increased my reach to 1172mm but results were poor :S. Till todate, teleconvertor is my only bad investment linked to photography.
Moral?? Save money and buy a telezoom lens like 55-250mm for Canon or 55-200mm for Nikon.
However, my experience with macro convertor has been totally opposite and very very zabardast results
I use Raynox DCR-250 for macro.