I have driven IST 1.3 (2003) for 3 years. Good car in term of space, maintenance and specially in fuel.. It gave me min 12 per litter in Colombo usual traffic, 14-15 outside. (Exceptionally 18 per litter in Kataragama with no A/C). Please be noted that I'm a feather footed driver.
Leg space for driver and front passenger is reasonable, however rear passengers might feel little uncomfortable due to short leg space. Parts can be found easily. Resale is quite fast. Had white color FL grade, nice sport looking and handy. Good pick-up in short drive (I usually get 200-300m lead after stop at color lights :P). Did no major repairs except replacing break pump, and usual running repairs such as break pads, belts, rear shocks, servicing ect..
Really good for daily city drives. I feel I missed my IST
Cons : For long journey, IST is not a good choice. The suspensions are stiff and can feel tired after driving. Steering is limited to certain level to avoid risky turning, so you may feel difficult to turn the car in small space.
Sorry no experience with GP