I've wanted to use this type of panel for years and finally bought them because I was replacing drapes and updating rods.  You could also buy the drapes that are already lined, but just as easy to use these, you can removed at any time if you do not need them.  I will say, they will add a lot of weight to your curtains and the rods will need to be securely anchored.  Other than that, the light blocking is great.  Can't wait to see if it also helps blocking out heat generated during the Summer months.