I have had six window glass units on the south side of my Anchorage, Alaska home "defogged". The process worked great for four of them, the other two were just too far gone and would refog every rain storm during the summer.
Now that said, I am getting additional windows defogged this year. The company up here will allow me to REPLACE the two glass units previously done where the treatment did not work, AND deduct the cost of the previous DeFog from the cost of the replacement glass.
Bottom Line: Defogging DOES work in MOST cases, and is worth the effort. Just ensure that your selected company will provide you the cost dollars BACK if you end up having to replace the window...UP in Anchorage they provide that warranty for 20 Years - Frankly if you have a good company standing behind their work, there is NO REASON Not to try it - For me so far it has saved from having to replace 4 glass units!