There was an article on this site that included talk about these awhile back (I think it was about deadlifting?). I gave them a try as accessory work some time ago and remember them being very difficult due to my tight hip flexors and weak glutes (Maybe I should do them more?). I think they could certainly be a solid addition, but it depends on what you need. I don't think they are intended to be hard on the low back, but that they would be if your glutes are weak and your abs are weak and your hip flexors are tight. Done with good form it should be a glute move primarily.