i've driven cng. there are a few factors you should take into consideration when installing in your petrol vehicle.
you loose quite a lot of boot space as you have to place the tank in the boot. say in the latest toyota corolla if you have a cng tank in the boot then you might not be able to fit in a suitcase properly ... if you can live with that then all good.
people look at the immediate benefits after installation ... like saving a dollar a gallon ... i'm not exactly sure of the cost in the US but in the uk its about £1800 to install and in pakistan it's about Rs45,000 for the whole kit ... if you drive 30 miles a day it would take you about 2 years to recover the initial cost then you will start saving a dollar a gallon.
next petrol burns at about 475 or 375 degrees farenheit and cng burns at 1175 degrees farenheight. basically 3 times the heat is produced in the combustion chamber having an adverse effect on the engine if it is only designed to run on petrol. i've notice engines running on cng require an overhaul before a same car running on petrol, thus spending the same amount again.
if you really want to save the environment and do not care about costs then choose cng. do not think of it as a money saver.