I never even considered using my 70-300vr with a teleconverter. I guess that Sigma does a fine job from what I'm seeing. Good to know there are TC's out there compatible. Maybe I'll have to check that out. I assume you lose the autofocus however no?
John - That's a great image. The background colors complement the wins on the butterflies so well. I have this lens and agree that it is a bargain for what you get with it. I also think another underrated steal is Nikon's 28-200 G lens.