I read Ishmael
several years ago. I thought it was an interesting idea and agreed with a lot of what was written in the book. I would like to go back and read it again some time and also read some more of Quinn's books. I recently read Jared Diamond's Guns, Germs and Steel
which gives a much more scientific account of human evolution from the perspective of biogeography. It would be worthwhile reading that book along with Ishmael
for another perspective.
However, it seems something of a cult has sprung up around Daniel Quinn, especially in the USA. People seem to believe that reading his books and encouraging others to read them is the key to saving the Earth from the impending ecological disaster.
You can check out Daniel Quinn's website if you want a bit more background information: http://www.ishmael.com/welcome.cfm
There was a film made a couple of years ago called Instinct
starring Anthony Hopkins and Cuba Gooding Jr. inspired
by the book Ishmael
(although it is definitely not a movie of the book).