I haven't owned a cng/lng vehicle but I satyed at a Holiday Inn last night.
But, seriously folks....
There's an ex-cop Crown Vic currently on ebay that is a CNG vehicle that I wanted to bid on so i did a lot,(and I do mean A LOT) of research on these cars.
There is way too much info to try to summarize so start Googling, but, in summary, here is what I learned:

1. As an alternative to gasoline, you want LNG (LIQUIFIED NATURAL GAS) aka PROPANE, not CNG (COMPRESSED NATURAL GAS). CNG is extremely hard to get. Ford the leading advocate of CNG in fleet vehicles gave up on CNG in 2004.
2. CNG tanks have an expiraton date and have to have periodic inspections. Buy a used CNG vehicle and it may be useless or be useless in the near future because replacing expired tanks costs thousands.
3. You can not operate a CNG vehicle on Propane. Conversion requires a degree in Engineering and Chemistry as I could find no co. out there that will do it.
4. Economics sometimes makes no sense in converting to LNG. Payback would take forever in normal use. Best for high mileage fleets.
5.Be prepared to give up a lot of cargo space.
6. You'll always have fuel for your tailgating BBQ.

I really wanted that CNG cop car but it would have been a really, really big mistake. No wonder the selling price is so cheap.

The miles I put on in a yr, even an LNG vehicle would never pay for itself unless gasoline went to 6 bucks.

Your mileage may vary...