I'm Czech and this revolutionary method of teaching came here in the 90's, when I entered my grammar school. Probably, it had a political “east vs. west” motivation among others, shortly after the velvet revolution...
The first thing I learned in the 1st year of schools were basic concepts of set theory. We were drawing circles, ellipses and Venn diagrams (even though we didn't call them like that) filled with images of apples, plums and cherries. Teachers explained to us what an intersection, union and set differences are and we were supposed to draw items into one set, but not in another set, etc.
I recall these exercises as funny and playful. They were similar to IQ tests in the sense that the exercise is logical, slightly entertaining, but highly abstract and loosely related to the world you know.
And I think this was the main issue. The 2nd topic we learned was simple arithmetics as in standard educational systems. However, at that time, I didn't see any relationship with the concepts of set theory.
Was the system any good? Hard to say. AFAIK, it was dropped after a few years. Eventually, I obtained a PhD in computer science, so at least, the system wasn't a complete disaster for me. :-)