I don't want to throw cold water (ha!) on the idea, but I agree with tophmck- be sure to have a way to remove it or cleaning your boil kettle will be a huge pain. So, you know, permanent-but-not-permanent.
I have installed my copper coils in my brew Kettle. It works fine and all I have to do to clean it is put oxy clean in the kettle and let it sit for a day or so. Have never had any problems with it. I have welded stainless steel fittings in the kettle and use hose fittings to hook up the coils. I do not take the out cause if you remove them you will have to replace the rubber seals on the hose fittings.