As one who tried the 'cheapo screw on macro lens (and telephoto,too!), I can tell you first hand they are junk!
I tried the screw-on telephoto first. My Canon 18-135 lens couldn't auto focus at all. So I resorted to manual focus. The resultant pictures were clearly not sharp. It might have been my manual focusing as well.
The straw that broke the camels back was trying to unscrew the telephoto...the screw on lens itself disassembled as it, too, was screwed together and contained 2 lenses and 2 spacers to hold the lenses in place. Picking up the components of the screw-on telephoto, they went directly to the trash...as did the corresponding macro lens, bought as a 'set' for about $40 as I recall.
If I wanted to 'get my feet wet' with macro work, I'd probably start with extension tubes. They're inexpensive and 100% foolproof. 35 years ago, I had some tubes and experimented with them, but found I wasn't terribly interested in doing macro work. I did, however, have an FD 50mm flat-field macro lens that I used several times to photograph documents and pictures at the Smithsonian Institution. The results were quite good, considering I couldn't use a flash.
