installation cost almost 3k. more or less but it must done the proper way by authorize center. (read on the paper about a waja taxi's blown up ngv tank before...    )
refill only available in kl and a few other big town. again, you have to fight for the refill with the taxis.
some petronas station refill more pressure and less gas, thus, return a lower mileage per ngv tank.
car feel less responsive with ngv too.
btw, you can always pump petrol once you are running out of ngv gas.

it's merely individual preference, go ahead and do your bit for the environment... 

I remember 1 case whereby the owner of the NGV cooked up in his own car, due to smoking inside which he didn't noticed that the NGV cable actually leaking and spreading the gas out.. Damn scared man.. 

The problem with filling is because it depend on pressure, if the  pressure at the station is low, it will take longer time for re-fill and it also unable to fill your tank 100%, thus lower mileage.
Usually because of improper installation or wrong equipment (use LPG tank to save cost). After installation you should go to puspakom for inspection, and you also got roadtax discount if not mistaken.