For what its worth...they aren't fracking for the natural gas. They are fracking for the crude. Natural gas is simply present in the wells they dig and they typically vent it or burn it off because they don't think its valuable enough to capture and transport it.....Sorry for the rant.
Other than that CNG is nearly 88% times cleaner than gasoline that regular vehicles are burning and it costs a fraction of what gasoline costs. Unfortunately the USA hasn't caught the picture like the rest of the world including the middle east in developing an ifrastructure to use it for vehicles. It is nearly as efficient if your vehicle is tuned for it and for the price a little less performance won't hurt me.
As far as mileage and maintenance goes on CNG because it is a cleaner fuel it causes less carbon build up for cleaner cylinders which results in a longer lasting higher engine performance for again less emissions. If you have a good cng kit installed it will run well and not have any major maintenance required. I have run CNG on both of my vehicles for the last 4.5 years and only had to replace a simple stepper motor on one system. (it had been on the vehicle since it was brand new. Over 10 yrs old) Also because it is cleaner you can run longer between oil changes. Typically you would need to perform the 3k-4k miles oil change and now my oil is still a light maple color at that point so i extended it to 5.5k miles and it still looks good.
If you are new to the CNG market and you are looking for a good and reliable vehicle. Look for a newer cavalier. They give you the benefit of a bifuel so you aren't left stranded if there isn't CNG available. If you want to go for an aftermarket CNK kit there are dozens of reputable kits available. I've installed several and they run great along with the benefit of running on both CNG and regular gas if needed. I believe the only draw back it the space the tank requires. It consumes your truck in a car and about the size of a truck tool box in a truck. Its worth it to me. hope this helped and good luck.
CNG Conversion Kits Info