The following video shows the Canon offering and conveys when and where this thing can be useful.
//youtu.be/http://www.youtube.com/watch?v=6LFiMKiOMgc
Unlike Canon; its cheaper, there are four lenses / filters that you can combine to get different magnifications and having respectable glass quality.
//youtu.be/http://www.youtube.com/watch?v=6uGTguJZtvE
Close Up Lenses / Filters are brand neutral; it will work with any lens having 77m diameter!