I sell teleconverters in our shop. People clap them onto the back of their telephoto lenses and head for South Efrica or the sports ground. They imagine that they are going to be able to get close-in to the players or the lions. In most cases they are wasting their time and money.
Teleconverters make the inside of the camera darker, and the mechanism that does the auto-focus cannot work well. In some cases it does not work at all. The auto-focus lens hunts back and forth and never settles upon a subject.
Even in the case of teleconverters where the lens is focussed manually, the end result is hardly pin-sharp and can have funny colours at the edges.
Buyers would be well-advised to get a longer tele-lens if they can afford it but to leave the teleconverters on the shelf.