You can definetly add value by applying some NN techiniques to your system. Basically, from my experience no single Neuraral Net to my knowledge can step forward w/o significant error. However, where it proves invaluable is tweaking the system that you already have, i.e. you have particular system that you're evaluating, and NN algorythm can be very timesaving as far as tweaking the optimal historical entry/exits relative to time/ volume etc. But as I said, i haven't seen neural network that would say S&P up 10 points tomorrow.