Sum in J is +/ not, $&$%^%$%M. Is +/ really too hard to comprehend? I agree with you that APL is historically important, but J is Ken Iverson's last and greatest gift to the programming community. He used the 26 years between the release of APL and J to address many of the shortcomings of APL. Take a look at http://bit.ly/hMmGWV for some background.
Personally, I found J a very interesting interesting exercise, and APL is historically SUPER important, but for practical uses they have been way superseded by matlab/ Octave/ R and the T/P/R* scripting languages. (At times my friend will say something like "in J you can compute the sum of a matrix IN JUST ONE LINE... and I will point to "sum(A)" in matlab, as opposed "$&$%^%$%M" in J, or whatever it is.)
K may be more brain friendly than J. But as far as I am concerned, J makes assembler and badly written Perl look easy to read.