I have a Canadian Cefaly, owned it for a few years now, and have extensive experience using it.
For me, personally, the "mode 2" (preventative) mode did absolutely nothing to help prevent eye strain. It did feel relaxing, and helped numb the pain when the device was in use, but it did not act as a preventative. I used it daily for 2 months, no preventative effect.
Mode 1 is a fairly effective pain killer for forehead strain when it is running, but it does not help the muscles around the eye itself where the eye strain usually occurs for me. It does help much more in the forehead and top of the head areas.
Not trying to be a downer, just sharing my experiences. I'm certainly glad I have it, and I do use it when I am triggered, but it is not a cure all or preventative device for me. It's a solid part of my toolkit when dealing with an eye strain migraine attack