Exactly what teleconverter model are you referring to, Davie.
quality of the optics, hence quality of the images you produce. You're place extra glass between your lens and the camera which will effect autofocus speed and reduce the amount of light by 2 f-stops reaching your camera. You don't have to buy the expensive branded TCs, there are some cheaper alternatives like kenko out there, but I would definitely avoid £20 glass