I took abacus classes when I lived in Japan for several years as a child.
It was interesting because one learned a mechanical way of solving arithmetic problems, but I didn't learn "number sense" which was much more useful later on in my academic career.
I think my experience meshed with Richard Feynman's conclusions in his excellent story Feynman vs. the Abacus.
How did the customer beat the abacus?
The number was 1729.03. I happened to know that a cubic foot contains
1728 cubic inches, so the answer is a tiny bit more than 12. The
excess, 1.03 is only one part in nearly 2000, and I had learned in
calculus that for small fractions, the cube root's excess is one-third
of the number's excess. So all I had to do is find the fraction
1/1728, and multiply by 4 (divide by 3 and multiply by 12). So I was
able to pull out a whole lot of digits that way.
A few weeks later, the man came into the cocktail lounge of the hotel
I was staying at. He recognized me and came over. "Tell me," he said,
"how were you able to do that cube-root problem so fast?"
I started to explain that it was an approximate method, and had to do
with the percentage of error. "Suppose you had given me 28. Now the
cube root of 27 is 3 ..."
He picks up his abacus: zzzzzzzzzzzzzzz— "Oh yes," he says.
I realized something: he doesn't know numbers. With the abacus, you
don't have to memorize a lot of arithmetic combinations; all you have
to do is to learn to push the little beads up and down. You don't have
to memorize 9+7=16; you just know that when you add 9, you push a
ten's bead up and pull a one's bead down. So we're slower at basic
arithmetic, but we know numbers.
All that being said, I'm glad I learned it, but it's useful now mainly as a parlor trick. I also demonstrate how to multiply with a slide rule! And it was interesting when I lived there to watch shopkeepers sum up checks.