I agree with Beabop. Even when they work, they are a lot of work to raise and lower. I have enough things that make my neck and back hurt, raising the blinds shouldn't be one of them.
We had to adjust the knobs all the time, only to have them fall down again the next day. And when you finally get the knobs tight enough to hold the blinds up, the string breaks, and, I don't think sitting and rethreading them with fishing line is something I want to "tackle"
... even with the excellent instructions found on this forum. That might keep the strings from breaking, but I would still have to climb over the sofa or dinette to adjust them.
Those blinds irritated me for almost 3 years. Now, I just LOVE my new mini-blinds. They were easy to install and are super easy to use. Keeps me happy....