John,
I have tried a cheapy set and the telephoto vignettes a lot, and the wide angle was crap on the edges, but I will need to look at the brand and see which one that was. However I did get some others but have not tried them, there seems to be a lot of variability for quality and a lot of variation for what people think are acceptable results. That is probably because my expectations are higher then most phone-togs. The cheapy set was more a clip on set and that was wobbly and did not grip well. The other set I got was from Beastgrip and I have not tried them but using the Beastgrip makes it a lot easier and more stable, but it add bulk. That being said I also picked up an anamorphoric lens from the Beastgrip people with the wide angle and have not tested it too much yet, but I think that might be an interesting thing. I have a desire to play more with video as well as stills and that aspect ratio is pretty interesting. Need to make some time for some fun....
Alan