Has anyone used these lifters ?

Variable duration lifters like Rhoads and anti pump up lifters are different animals kinda at opposite ends of the spectrum. People sometimes think they are the same. They are not.

Also, Rhoads are different than other variable duration lifters, because they do not use oil pressure to change duration. That feature is handled only with RPM. Oil pressure has nothing to do with it. Go on their site and read about them. It tells you all about them and how they operate.

I've never been a fan, but they do work. If I want a cam to "act smaller" I'll just choose a smaller cam. I know, the Rhoads lifters give the benefit of having the cam "big again" at higher RPM, but there's that little voice in the back of my mind that's asking if they are allowing ALL of the cam's lift and duration to be given at those higher RPMs. There's really no way to tell. That's why I have sworn off hydraulic lifters for my builds and will only use them for someone else. Just my personal opinion.