I use one and think it's great. Not only does it prevent wear and tear on your string, it also (and more importantly) allows you to draw from directly behind the arrow thus preventing arrow torque and nock pitch. IMO, this makes the bow easier to tune and improves accuracy.

Great invention and one of the best things to come along in archery in some time. While other gizmos and gadgets aren't worth the plastic their packaged in, this painfully simple design is one of the best upgrades you can put on your bow.

I wish I'd have thought of it.

My .02