I made this Tau Ethereal from an Island of Blood High Elf Mage. I traded for the base mini on Bartertown for a song ($5 if I remember right) and uses a Fire Warrior head. Carving the Elf head out was harder than expected and if I was going to convert the mini properly, I would have shaved off a finger of each hand to make it a real Tau. Unfortunately, that idea did not occur until I was already painting.
As a conversion it works great. The only mistep that is not correctable is the magic smoke that the Mage is standing on. Tau don't cast any magic and him standing on that crap only make him a better target.
I had some problems deciding on how it wanted to paint the ice blue that I painted the rest of the Tau army in on this Ethereal. I did not know how white or how blue I wanted the fabric and went back and forth several times. I think the end result does not look unduly out of place next to the other finished Tau though.