what is it? Some claim they get good results, some seem to describe them as an abomination. Obviously the truth is somewhere in the middle, but... where? I was looking at getting a 1.4 TC but the lens I'd use with it is not top notch to start with: it's a sigma 18-200 and it's 6.3 @ 200; would that even make sense? it would be to use in bright settings though.