There's no question that DEET works, but aside from the potential threat to human health, you should also be aware that there is a SERIOUS threat to the health of your gear.
I have some stuff that is 100% DEET and I made the mistake one evening of spraying it on my forehead while wearing a headlamp which is made of a hard PVC outer shell. Well, about 30 minutes after doing this, I reached up to turn it on -- it had become dark. I felt something very gooey and wet up there. Upon inspection, I found that the DEET had melted the plastic nearly an 1/8" deep.... you may be saying to yourself, "Big deal, so I won't go night fishing when the bugs are out." But guess what else is made of PVC plastic: the outer coating of your fly line!! Yes, you could easily melt entire sections of coating off your line with a single spray. On the bright side, DEET would work really well in preparing a line for a blind splice
Another thing not to get DEET near is your waders. One test of how good waders are is how well they stand up to a 25% DEET solution. The best out there still suffer some damage. The worst will leak as a result. I can only imagine what a 100% solution would do.
So, if you do opt for high concentration DEET products, please use with caution. You could end up getting bitten where it really hurts: your wallet.
tight (DEET free) lines,