Personally I dont think they are worth the trouble, unless you run alcohol, and have a drag motor. (if you are drag racing id say its a must, if you run alcho also say its a must!)
some people will prob chime in and say they like them, If not setup correctly though, you will melt your motor!
setup requires a dial indicator, and is not as straight forward as you would hope.
(very well built german engineered product, with poor instructions)