I cover football matches and you generally need at least a 70-200mm f2.8 lens to get decent images, though even then the lens is not long enough for some shots. With the teleconverter, whilst I lose one stop of light and the lens becomes a f4 lens, its worth losing the light to get a 140-400mm lens.
I used the lens this week on a floodlight FA Youth Cup game and though I had to use a high ISO the image was pin sharp and the lens focused as fast as it would have without the teleconverter. This is a hell of a test for a teleconverter and it shone.
Recommended if you can afford a 300 or 400mm lens as this gives you the range with spending £2,000 upwards.