im not going to bad mouth the stuff buti have some opinions on why i would never run it
1-methanol (itself) burns clear and that scares the crap out of me
2-i never like being at the "limit" of anything in my car. for example, you can run a TON of timing with this kit, but what if something fails? then you are at a huge risk of a boom. i use the same mentality when i sprayed my car. I ran extra fuel pressure and retarded the timing a few degrees. neither was 100% necessary but i would rather have a safety cushion while walking a thin line.
3-they pretty much say it in their site
i like to keep non flammable substances out of my engine as often as possible.
4-They call it "an insurance policy." But if thats the only thing that is allowing you to keep your motor from not blowing up, then I call it a "band aid". Its something i personally would never use