Your thoughts on a 2X TeleConverter?

BrianA2 wrote:

I am interested in this discussion because I am looking into the possibility of a 70-200 f2.8 + 2x teleconverter combination that works properly, including autofocus.

On a recent trip to Madagascar, I found myself with a very non-optimal lens setup. I took my Bigma and k-x, but found that when photographing lemurs in the forests, most of the time the distances were not great, but the light levels were poor, so the 70-200 2.8 would have been a better choice. However, there were some occasions, particularily for birds, where the longer focal length was useful, and the light was ok.

The sigma 70-200 plus teleconverter combination is lighter than the bigma, which would have also been an advantage as the terrain was often difficult and most shots were hand-held. So I gather that what I need is an old sigma 70-200 (I assume series 1 rather than 2) and a sigma 2x. Any other combinations that anyone has used successfully?