diesel mileage is pretty good - about 15 km/lit on normal use in a toyota 2C - but it all depends on how is the engine adjusted and tuned up, Most mechanics dont know jack about this in Pakistan and simply say - over all kardo - more money in their pocket and less money in yours.
They require heavy duty batteries to start - so its vital to use a proper battery prepared with pure electrolyte. If you can find a sensible mechanic then its pretty good, otherwise you will become a poor man soon.