Since the end of 2004, when the book was first published, the Magic Formula as described in the book has returned 12.68%/yr (CAGR) while the Vanguard US Total Stock Market Index has done 8.1%/yr. Although that may not be near the outperformance as originally described in the book, it has still theoretically beaten the market.
I took the data from the Little Book that Still Beats the Market that updates the test from 2004 to 2010, and I've been live tracking the strategy in a spreadsheet each year since then. Although these are not results from a real money portfolio, you can do with it whatever you want.
That being said, if you aren't 100% certain that it's the strategy for you, then of course you should not follow it. You will not stick with a strategy if you don't believe in it. This is why Joel Greenblatt (who I personally believe he's genuinely trying to help the little guy, he doesn't need our money, trust me) started his mutual funds, because the people who tried to follow the strategy on their own inevitably underperformed on a consistent basis. It was their own emotions and psychology that kept them from following a black and white rules-based strat. He found that people made much more money by letting Greenblatt make the trades, even though theoretically they should have performed the same.
So, IMO, the problem is not necessarily the strategy. The problem is you. You should find something that fits your personality and that you'll stick with it no matter what.