Has anyone tried these yet? http://www.auroraballast.com/
It's a fairly new product, and from what I understand, they are working on expanding their product line. We retrofitted one of our smaller buildings with these as an experiment. We went from a 277vac, 250 watt HPS, to their 320 watt MH. These are high bay fixtures. The ballast is extremely light, and was easy to install. I hope they come out with different wattages and voltages soon.

Guess not

Are they quiet?

very

I have used a few of the new 39 and 70W ballasts designed for T6/PAR ceramic metal halide lamps. They have the size and look of a CFL ballast. They were great to work with and I'd use them again. Only poblem is your lamp lead length is very limited, so be careful w/ remote ballasted fixures. The old core and coil ballasts seem very old fashioned these days. I think eventually they will go the way of the magnetic flourescent ballast, relegated to home depot fixures. Now if we could only phase out the T12 bulb!

I believe the units I dealt with were form Advance: http://www.advancetransformer.com/products/hid-electronic.jsp