I have been using Neem oil as an insecticide on and off for years.
It sticks to the leaves and continues to work for some time after spraying. It is especially good to repel white fly which congregate on the underside of cabbage or broccoli leaves.
Recently I came across this book, Neem: A tree for solving global problems which was published in the early 1990's by The National Academy Press in Washington D.C. It is a comprehensive guide to the use of Neem worldwide up to the date of publication.
Now it seems that the current opinion is that the Neem tree has serious weed potential in most regions of the dry tropics where it has been planted, including Australia. Birds find its fruit desirable and thus spread the seed far and wide.Pest plant risk assessment:Neem tree—Azadirachta indica - IPA-Neem-Tree-Risk-Assessment.pdf