I'll just pop my two cents in this thread.

It is my belief that there is potential to profit from following a mechanical trading system religiously, (i.e no missed trades) but I personally have found after years of both backtesting and live (forward) testing of numourous machanical trading systems that your returns are likely to be rather lack lustre in nature with quite some volitility/fluctuations (and occasionally some stomach renching drawdowns periods) in your equity graph.

After looking over my trading results (both Live and demo) and findings to date, I've largely came to a conclusion similar to what keris2112 has outlined... That one needs to apply discretion in their trading and be able to make emprical decisions in order to achieve anything more than lack lustre trading results. Achieving the experience necessary for discretionary trading I believe will require a number of years of ACTIVE experience in the Forex Markets.