Bumping an old thread but I'm too lazy to look up a newer one on the PP.
After many years of investing in a standard 4x25 PP strategy, I'm a confirmed fan and recommend it to friends and family for both accumulation AND wealth preservation in retirement. The best description I've seen is that it's like standing in the eye of a hurricane; in any given year 1 or more piece is usually down double digits, but another asset more than takes up the slack, and the net result for 40 years has been a nice smooth upward trend. Rebalance at year end, rinse, and repeat. In my experience most of the criticisms people make have already been anticipated and answered in the literature:
"The PP is just another faddish strategy designed to sell books": Nope. Just go to crawlingroad.com and you can find all the information you could possibly want for free (there are books for those who are interested but they are totally unnecessary as the strategy emphasizes simplicity and ease of use). As a passive strategy utilizing low-cost index funds, transaction costs and expenses are kept very low. No one is getting rich off selling you the PP, and the website is bracingly down-to-earth and free of hype or salesmanship (I love his random post about how he hates tapas).
"That's too much cash and not enough stocks! You are giving up a lot of return over the long haul": The long-term average annual return of the PP is about 1% lower than the S&P 500, but it has orders of magnitude less volatility. On a risk-adjusted return basis (which is what really counts) the PP absolutely crushes the S&P 500. People like stocks because they can increase by 50 or 60% in a single year and it feels like you're getting rich overnight, but they can also fall by half in a year (or 20% in a SINGLE DAY). People with stock-heavy allocations felt brilliant earning 30% in 2013, and horrible losing 40% in 2008. Who needs that kind of emotional roller coaster when you can get basically the same long-term return with 1/4th the volatility? As for the cash, it's essential for smoothing out the volatility, plus it makes a useful emergency fund. Yes, there are inflation losses, but it's the only asset with 0 risk of capital losses, and diversification is all about mixing sources of risk.
"PP historical returns were skewed because gold/bonds/whatever had a decades-long secular bull market that won't continue": Maybe so. If you have strong opinions about how this or that asset class is hugely overvalued/undervalued or this or that trend is due for a reversal any day now, I wish you luck cashing in on those insights. The vast majority of people who try, fail (including rocket scientists and Harvard grads), but maybe you are the exception. Certainly you will have a white-knuckle ride watching the market every day to see if your theory is panning out, and agonizing over whether to cut your losses or lock in your gains whenever a big move comes along. For people who believe the future is chaotic and unpredictable and just want to earn a decent real return over time with no headaches, all you have to do is diversify broadly and trim the winners/top up the losers once a year without speculating over what next year's winners/losers will be.
One thing I really like about MMM's writings is the emphasis on simplicity and developing a good underlying philosophy to guide specific actions. I have always been a little uneasy with his recommendation to put most of your savings in stock index funds though, since that simply doesn't provide the stability needed to properly support early retirement (or else you have to accumulate twice as much wealth to allow for the fact that you could lose half of it in the blink of an eye). The PP is rooted in long-standing common sense about investing: diversify, don't time the markets, keep fees and expenses low, stay the course over time, and don't expect your portfolio to make you rich overnight. I say this as a professional institutional investment manager and CFA: the PP is a great choice for retirement savings for retail investors, especially ones who prefer a simple, low-stress, hands-off approach.