I love them!
There are at least a few threads in the Main forum; if you look at them you'll find lots of opinions and information.
The Slidelock doesn't go on top of the ruler, it butts up to it. I did try it on top of the ruler, and as expected, it didn't work as well that way. The balls that engage to touch the fabric are what holds it - if they are not against the fabric, they aren't going to hold it securely. They would be against the ruler, which might not slip away from the Slidelock, but the ruler's other side would still be against the fabric with nothing to keep it from slipping.
Here is one of the other threads
http://www.quiltingboard.com/main-f1...t-t239186.html
As far as the other ruler grips - tried them, hated them, gave them away.