I've used the HD650s for mixing a lot and I've had good results. I chose them because they sounded clearly more balanced and detailed than any of the other headphones I auditioned, which I think was the Beyerdynaimcs DT-880 Pro and AKG K-702 models. As I recall they sounded a lot more like what I was hearing from monitors (Dynaudio BM6 and some high end Genelec). Just more natural.

The main thing though, for me, is that they sound splendid and are quite detailed which make them a pleasure to work with. So far, anything I've made on them that sounded good has sounded good on other systems, so I can't complain about their performance for actually mixing either.

I don't have a lot of experience in mixing though, and have only been producing my own demos with them, so for higher level skills/tasks maybe something more honest could be easier to work with. They do have a potentially disruptive peak in the higher registers but their bass response and mid range definition is great.

The only other phones I've actually worked with myself are K-702s, and I much prefer my HD650s, for both comfort and sound.