The availability is a bit limited now. And, you'll need a car fueled by CNG, like this one:
The combination is great for a commuter car. But, you would have to compare the cost of both, plus the price of natural gas, to buying something like a Prius that gets better mileage.
Doesn’t the Phill have a terrible reputation for poor reliability / short life?
I remember seeing a lot of complaints on a forum that was mostly California folks.