I've used it for a couple of test (i.e. not paying!) shoots and it's an interesting modifier, but like most things, it has its limitations.
1. It *eats* light. Think +1 EV at the flash and about +1.5EV on the camera as a starting point. Adjust to taste.
2. Effective range with an SB 910 is about three metres. I'm wondering about a way to stuff a Quadra head into it without frying the whole thing. Probably not a good idea...
3. You've got to use it close in if you want those lovely round catchlights.. Headshots at 70-80mm are well lit, but the catchlight isn't obvious any more.
4. Kiss goodbye to your AF assist.
As soon as I have the measure of it, I'll post some of my results.
Thanks Charlotte. Sounds like a one-trick pony, but I rather like that trick
I think it's unique this side of DIY jobbies. Look forward to your results - please post a link here or I'll miss it in the portraits section
Power loss sounds like a bit more than two or three stops
but I think that's the price for even brightness around the ring, and the size. Rayflash/Orbis etc can't match that. Even if you could get a Quadra to fit, suspect you'd lose the nice even ring.
Edit: not sure if this might help your exposure issues, but with Canon guns, when the head is tilted or rotated, it disables the distance data that the camera uses as a major part of TTL exposure calculation at close range. Of course that gets screwed with accessories like the RoundFlash. In fact with Canon guns, all you have to do to disable distance data is push the tilt/rotate lock button.
Another thing, some of the DIY ring lights of this kind of size, using LED or small fluorescent bulbs, create some some cool catch-light effects, eg
http://www.pxleyes.com/tutorial/photography/2358/How-to-Make-a-Cheap-DIY-Ring-Light.html You could do that by cutting holes or other shapes in some kitchen foil and attaching to the front. It wouldn't help the light output, but with silver foil reflecting some light back in, it might be workable
