There's a very interesting article to that end at Climate Progress. Joe Romm says that without carbon-dioxide pricing, i.e., things such as a tax, and, without taking more steps to cut methane leakage from pipelines and other spots, it's probably only about 20 percent better than gasoline/petroleum as a bridge vehicular fuel and longer-term bridge to replace coal for electric generation.
Now, it is by Joe Romm, who is somewhat apocalyptic on climate change issues. But, the leakage concern comes from an outside journal, first. Second, the worry that we will eventually get a bunch of LNG or CNG vehicles rolling off the lines in Detroit is a legitimate one. Obama's tougher EPA CAFE standards have loopholes for alt-fuel vehicles. I expected Detroit to look at E85 cars given the government's push for more ethanol, but, the natural-gas angle makes sense too, since fracking of shale fields is spreading to more and more states, and exploiting the loophole would scratch a lot of backs.
So, give it a good read. It may not be too apocalyptic.