They were actually tested and compared against commercial brands. Tile and linoleum floors, formica counter-tops, appliances, etc. Read about ammonia above. The best commercial preparations left the window only a little shinier. Even though the cornstarch makes the mixture slightly gritty, it didn't scratch the glass.
Ammonia is poisonous, so keep the mixture away from children and arrange good ventilation.
Wear gloves because it's a heavy-duty cleaner and rough on the hands. Try not to clean glass the sun is shining on because it will dry too fast and streak. We'd been warned that plain water could do as well as a vinegar solution, but our subjective impression was that the vinegar made it a lot easier to get rid of smudges. In theory, vinegar is supposed to remove hard-water spots. May be hard on your hands, but safe enough to drink.