I've used it with an old Kenko MC7, which was ok, decent center sharpness, significant CA, but wasn't 2x, probably 1.85x. Then I tried a Kenko MC4 DGX, which was not very good, not better than the MC7. Then I bought my current Sigma 2x APO EX DG. It was much better than the Kenkos, very sharp, average amount of CA. Then came a Canon 2x III. It was nice, good center sharpness and relatively good corners. What was best about it was the lack of CA, that thing handles fringing extremely well. But it wasn't sharper than the Sigma , in fact I had more images where the Sigma had a tiny bit better center sharpness than the Canon III. Corners were a bit better on the Canon III. CA was visibly better on the Canon III.
But then I needed money and so I sold the Canon III and kept the Sigma, which overall I very much like. Recently I've purchased a Sigma 1.4x APO EX DG and that is awesome, surprisingly a bit sharper than my Kenko Pro 300 DG was, and I thought that was a pretty good tc.