I've never used one of these, although I plan on it at some point. I can do a the research myself, but if anyone has a strong opinion on one product or another, let's hear it.
But regardless of the system, my question about these is this, and I'm not sure why I haven't heard any mention of this one way or the other. Let's just assume a straight square pattern. Normally if there is lippage, I would do one or both of
- push down the high corner of a tile, embedding it further into the thinset, or
- lift a tile and add thinset under a low corner, ensuring that the corner were fully supported.
The problem I have with these tile levelers is that, in theory at least, you are going to be lifting a corner of tile up away from the thinset, and that corner of the tile is going to be unsupported. Isn't that a really bad thing to do in theory? Maybe it's not an issue in practice because it's difficult to stand on that little corner of tile and put much pressure on it because you're really standing on the other 3 tile corners at that intersection, which are presumably fully supported.