Like most traders who have operated as both a discretionary trader and a quantitative or algorithmic trader, I have spent a lot of time thinking about the pros and cons of both systems. Currently, I am almost 100% algo driven. This story gives data on both styles and shows the edge that systems traders have.
http://www.etf.com/sections/index-investor-corner/swedroe-problems-formulaic-value-investing?nopaging=1
If I had personally had the discipline to stay as a system trader throughout my career I would have done much better. But following a system day to day can feel very stupid. Because the system will have you entering positions when the market feels very extended one way or the other. Your mind will tell you you know much better. But our minds are programmed to lose money.
We are easily convinced we know THE truth when actually what we know is A truth. And we catalog the things we think we know rather than the things we surely do not know.