I really aprpreciate friends comments here. It is becoming a knowlege base and other people will get benefit of it.
I have few comments as I am researching on this now a days:
1-I visited two rent a cars and specially asked about CNG issus in 2009, 2010 and even 2011. Most of people (drivers) said that they are running these cars over 100,000 kilometers and there is not a 1% issue in any car CNG
2-Regarding consumption there is lot of difference while running car on Petrol in city. Like 150 kilometers are coverted in 55 cylinder in Rs 550 - 600, while 150 kilometers on petrol are covered in Rs. 1500 (with avg of 9-10 KM / L). So it makes 1:3 ratio.So I think CNG is lot economic than petrol in city. However in long driver this differnet comes to 1:2 ratio.
3-I am still looking for more people who are using CNG in GLI's so that we can learn from their experince.