I've been driving a Nissan AD since January this year. I haven't noticed any problems to date, I think it's a great family car and I would definitely recommend it to any potential buyers.
Moving on, I've installed a 55kg cylinder coupled with a Landi Renzo CNG kit and a PVR emulator. So far, I haven't felt the need to install a spark advancer as my car runs fine even without it. The check engine light comes on every time I run my car on CNG and when I got it checked, the code that was shown turned out to be "lean fuel mixture". I get a feeling that installing a spark advancer would resolve the issue.
As far as mileage is concerned, I get the following (keep in mind it has an auto-tranny):
City with AC: 120 kms
City without AC: 140-150 kms
Highway with AC: around 170 kms ( just a rough estimate, haven't checked as yet)
Highway without AC: 190-200 kms ( 191 for Motorway, and 202 for G.T road)
On petrol, my car did around 12 km/l within the city. I have no idea regarding the figures for highway driving as I haven't had the opportunity as yet. On petrol, that is
Regarding the availability of spares, there are a couple of shops in Chaah Sultan that I've personally visited that have stocked body parts for Nissan AD. I even have there visiting cards. Engine and mechanical parts are readily available in Saddar.
The only drawback is it's rear seat. Yes, it is uncomfortable and it cannot be ignored. I'm considering different options to take care of the above issue, and I'll definitely let my fellow Pakwheelers know whatever I come up with.
Hope it helps.