Oct 2011 update: Well, it seems that *our* nits have became resistant to this solution, I left it on my daughter's hair both 15 min and overnight and I found them still alive when I proceeded to comb them out. Time to change it, but the tips bellow still apply.
I can't really agree with the previous comments because this product worked for me everytime I needed it.
I have 3 kids in school so you can imagine nits are a normal occurence in my household, much to my distress.
The reason lice return within short time of first treatament is because eggs are not killed in the first instance. ALWAYS and I mean always, you need to apply solution a second time, after one week of the first application. Even with the solutions available previously on the market, not all eggs were eliminated. This meant that whatever eggs managed to escape, they hatched and started the life cycle again.
In order for it to work, it is important that the life cycle is destroyed.
So, first kill all the live lice by applying solution first, then after a week, when the rest of the eggs have hatched, apply solution once more to kill the remaining hatched eggs, before they have the chance to start a new life cycle.
It is troublesome, but this is the best way to get rid of lice.
This is what I do, and it works, for a while we are nit free, but of course, kids will come back again with them, that's life!
A good product that acts fast, is non toxic and is well tolerated.