From my understanding, double glazing is great for thermal insulation with air gaps of around 10mm.
However, for effective noise insulation (my primary concern) you need an air gap width of 100-120mm for best results.
I have found companies such as http://www.magnetite.com.au and http://www.stopnoise.com.au who supply and install two types of secondary glazing.
Does anyone have any experience with these products or secondary glazing in general?