This article suggests that
This article suggests that NGVs cannot currently be filled quickly. This is true of home filling dispensers but not of public filling stations. It takes about 5min to fill a Honda Civic CNG at a public CNG filling station. Why? Because at the public filling station, the gas is already compressed to 3600psi; the customer is merely transferring it to the vehicle. By contrast, at home, with a 'time fill' cascade compressor, one begins with low pressure city gas, and compresses on the spot to the 3600psi required. It therefore takes much longer to fill the tank.
It is unlikely that many people will want to purchase compressors. But it is nearly certain that diesel trucks will be switching en mass to CNG. This is beginning to drive the construction of public stations, and once those stations are in place, owners of private cars will follow. Oil is too useful and too expensive to be used for transport when a much cheaper alternative is available.