If one made regular monthly investments into Series I Savings Bonds since the program began in September 1998, you would have done better than if you did the same monthly investment into a S/P-Index fund. Tom Adams at Savings Bonds Advisor has been plotting the performance
of both for seveal years. The index fund performance is currently just below the line that plots a zero return.
Unfortunately, the I bond base rates have been very low in the last few years. Tom compares the I Bond base rates with TIPS in this post.