I upgraded from a 2002 City EXiS to a 2007 Reborn a couple of years ago. There is a huge price difference between early model and late model Reborns. Following are some of the pet problems associated with early model Reborns which I myself encountered:
1) The right engine mount goes bad. Mine used to give a bit of a vibration so I pretty much replaced that mount soon after buying
2) Power steering leakage. Mine used to drip a bit of oil on the floor (but only required topping up every couple of months). Anyway, just because of the mess it used to create, I had the pipe replaced a few months back
3) The serpentine belt tensioner goes bad. Mine didnt display any symptoms as such but I just changed it for the heck of it (a few months ago) and the engine now seems to have been liberated and is so much more responsive
Other than that I would say its the usual stuff that you look for in a car. The worst things that I can think of are the hard uncomfortable seats and poor mileage. Setting this aside, its an awesome car, great fun. Here in Karachi I guess you can get a very decent 2007 Reborn for around 1.1million. I have no experience with CNG and always buy cars that have been driven on petrol.