I-bonds are unique and pretty interesting investments. They have zero volatility, an inflation adjustment, and you get to pick your own maturity date -- anything from one year to 30 years. And they have some tax advantages to boot.
If you're looking for preservation of capital, they're hard to beat. If you're looking for growth, they can't compete with stocks.
Personally, I'd buy all of the i-bonds the gov't would let me have at any fixed rate over 2%.
The interesting little bit of data that apparently prompted this comparison is that if you invest in stocks at high P/E levels, your 10-year return could be as low as 1.24%, and generally they found the risk premium to be only 1.32% over t-bill rates (which currently have a negative real yield). So, in some cases, stocks give you the worst of both worlds: low returns and high volatility. This is obviously a Bad Thing for any investor, but it's murder to a retiree.
The stock market is currently in a high P/E range according to the paper (high > 15).