I've used them, but only once so far, in drizzly but windless conditions, so I can't tell you how well they hold in windy conditions. I was curious about them, and don't really like wrestling with knots, so I bought some to try out. I put two of them on some 2 mm line I had sitting around. I'm not sure what the line is made of, but it isn't slippery. I don't think these rings would work as well on a slippery line, but they were very easy to use and held very well on the non-slippery line.
The rest of my lines came with my tent when I bought it second-hand, and they have rectangular plastic tensioners on them. Those work well enough but I find them cumbersome to adjust, since they don't really slide. I like the MSR Cam Rings much better, and will probably replace the plastic tensioners with them (as well as replacing the lines themselves with better stuff).