Yup, I have. Sold the TC! Snappy it wasn't. I believe that the Mark 3 TC is better than my Mark 2, so I'm thinking of renting one for a weekend to test it out: it's a lot of money to lay out only to find it doesn't do what I want.
There was nothing wrong with the TC, but it just wasn't doing what I wanted - that may be my fault!
My Mark 2 was OK in very good light, or for static objects, but part of the attraction for me of the MkIII was it's ability to capture BIF. It does an excellent job with the 400, but with the TC it was a disappointment. In less than perfect light BIF were almost impossible.
Others may (probably will) say different!!!