Yes, there is a lot of bad-quality stuff on the market. This is important: "Numbers 3T, 4T, 5T and 6T are two-element achromatic lenses that still maintain a high image quality. 3T and 4T work best with lenses from 85mm to 200mm, and 5T and 6T with lenses from 70mm to 210mm." I also use them for wide-angle lenses by the way were the effect may be less, but still is there, especially when looking for the best background.
Disadvantage is that the usage of close-up filters are not in the EXIF, unless you store it in the 'comments field' or author.