I have not tried this product, but I don't like the design for two reasons. First, IMHO an anti-cant indicator should be mounted to the scope, not the rail. There are a couple of reasons for this. If it's attached to the scope, then it doesn't need to be realigned if the scope position is adjusted or the scope is moved to a different rifle. Also, if the scope is mounted offset from the receiver, the proper alignment of the reticle is through barrel bore, not perpendicular to the rail. That alignment eliminates range-dependent aiming errors.
Second, this design puts the level too close to the eye and under the scope where the lighting is poor. I like the level further away where it is easier to get in focus, and out where it can get good light. I also like it on the left side of the scope so that I can view it with my left eye while sighting with my right.