I have a Vivitar version of something similar for my Olympus OM equipment, and I agree - they work very well.

It is a quite decent 2x tele-converter at normal distances, and then transitions seamlessly into an adjustable extension tube that gives life-size results.

I've never evaluated it for flat-field performance, but for really flexible close-up work it gives very good results.

And it converts my 50mm f/1.4 into a 100mm f/2.8 - nothing to complain about there .