I've had this for a few weeks now. Its fantastic. Been using it with the 400ex and it works ok with that. Line of sight is weak but it works. I'm mainly using it with my skyport. Getting some fantastic results. Love the clip on colour filter too, I cut out som card for it and now using it as a defuser, works great and will save you a penny or two buying a separate defuser.
Well done Canon, love it!