Diesel plus CNG

I'm being told by a D**ge guy that they are using Compressed Natural Gas with big savings on overall fuel costs, anyone doing that with a Duramax?
I know Mark runs propane, but has anyone tried CNG, which is supposedly easily available and safer? What he says is that they just feed it pre-turbo and let the turbo take in as much as it needs which makes the injectors cut back on the amount of diesel required. It makes sense, but has anyone tried it? How well does it work? Is it worth the expense of the tanks and plumbing, assuming access to CNG is not too difficult.