Very accurate - but only for things that don't contain a free food - chocolate, biscuits, cake, crisps, alcohol etc.
To find the syns, if any, in products that contains a free food, you'd have to look in SW books or on-line. The newer products that aren't listed can still be calculated on the SW Calculator if the NV is supplied - Calories, Protein, Carbs & Fat. The amount or serving size also needs to be provided for accuracy.