I bought a secondhand KB100-4S in 2000 and used it for two years during my PG. The fact that our institute was located almost 25Km outside the city and it was almost 2-3 Km between the hostels and the academic area meant a lot of riding was done in the two years.
The machine of 1994 vintage was found to be extremely enduring. The milage was among the best on campus and the pickup was fairly decent. But being a light bike there was little confidence while driving at high speed as it used to shake a little bit. But apart from these hiccups it was a very good ride.
On the plus side: Good mileage, decent pickup, comfortable ride, lightness and good handling.
On the minus side: High maintenance, rather delicate framework and low stability at high speed.
All in all its a decent bike which one will not regret owning if constrained by budget. and of course the savings are dual initial price and fuel costs.