indeed its environmental friendly but not engine friendly . there are alot of myths on this,
so its better is to stay away from it.
my personal view on cng is that i dont like a 60KG bomb sitting in my trunk, dar lagta hai . so i calculate and limit my drive and save fuel cost.
secondly being a dry fuel the engine heats up alot which in longer term damage the gasket, valves, injectors etc. so eventually you will end up paying more than your saved amount.
above facts are based on my basic knowledge.