Cam Guard
I have been using it for several years. I can't point to anything definitive but for the corrosion protection that they claim, and what objective testers say it provides, it is worth it for me.
Less definitive, and I am the biggest skeptic you will meet, I think my oil consumption went down, the oil seems to stay cleaner longer and there is less of it on the belly. IE, less blowby. It definitely changed about the time I started using it but I had done a new top about 200 hours earlier and I suppose it could have finally fully broken in about the same time.