I've been using i-Cuff for a few years and it has to be one of the best things around, much better than a chamois for comfort and blocking outside light.
Go get a roll of tool box liner, the stuff mechanics use to keep their tools from sliding around. It's a soft rubber-like mesh, then just cut a 1"x3-4" strip, and put it around the finder first, then put the icuff on over it, it'll never slip again.
There is no comparison between products. The chamois merely comes in contact with your face. The i-cuff wraps around your face and also works for folks who wear glasses. Also, i-cuff is breathable, washable and waterproof. It minimizes fogging in the viewfinder and has comfortable ultra-suede that comes in contact with your face. It will provide over 100,000 rubs before breaking down. Also, if you rotate i-cuff 45 degrees counterclock it will act as a hood when shooting away from your face under arm, down low, or overhead. Regarding tightening it down on viewfinders that are slippery; ie: no-rubber parts such as metal, plastic, etc. The above note from (when I was young) regarding the non-slip material is a great idea. We have suggested using the fat rubber bands that come on broccoli from the supermarket to act as a non-slip way to anchor i-cuff. Also, many folks are now using i-cuff on everything from binoculars, microscopes, spotting scopes to gun scopes, and telescopes Visit our site: http://www.i-cuff.com