On the plus side, you could do many three significant figure calculations
(OFTEN 'good enough') as fast or faster than with a modern calculator, and
RARELY made the common 'multi-order of magnitude' mistakes that now plague
our students. They just don't THINK. Whatever the calculator says, they
take as gospel. We see 'em calculate the size of some sub atomic structure
such that it wouldn't fit between the Earth and the Moon! Garbage in,
garbage out ... but FAST, and to ten figure accuracy. Ignorance is bliss.
The concept of the quick, in your head or on paper, order of magnitude
calculation is all but lost.

Dan Mitchell
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For example, I was taught by one high school teacher to estimate in your
head first then use the slide rule or calculator for a final figure. For
example if you're multiplying 11.31 x 36.416, first estimate in your head 10
x 36 = 360, then your answer can't be anywhere near 3600 or 36 or 360000.
That helps a lot.