i got my City 2005 IDSI on CNG and faced no problem at all. Car runs perfectly, in normal driving you dont feel like its on CNG. Mileage is like 150 to 160km on 55 KG Cylinder within City. It was normal Landirenzo kit with PVR Emulator+Advancer. CNG mainly depends on installation and if done properly will not give u any problem.
Biggest TIP for CNG: If it is tuned properly dont ever mess with it!!!