No experience with it, but personally, it's not where I want the level. It's too close for my eye to focus on, and I prefer that the level is positioned for my non-shooting eye.
I like a level mounted on the scope tube, offset to the side of the non-shooting eye, and forward of the turrets (often forward of the front ring). For a right-hand shooter, this allows the level to be seemingly superimposed into the upper right quadrant of the scope's sight picture not far from the center. After getting used to this setup, it works very well without drawing the shooting eye away from the reticle (and target). YMMV
http://www.midwayusa.com/product/846101/hollands-signature-series-anti-cant-device-30mm-matte