Fantastic discussion, thank you to everyone who's contributed.

For my money any potential investment has to compete against the risk-free return rate.
So as of today, you could purchase a 1Year T-Bill that will return 4.93%, a sleep easy, boring, (I like boring!) no stress return.
Or you could purchase the S&P 500 with a P/E of 21.8, and an earnings yield of 4.59%.
Of course the decision is yours, based on your need, willingness and ability to take risk.
That said, stocks are pricey, no one can deny that.(Even in this thread we've been given examples of why stocks should be pricey.)

Valuations do matter for the long term investor'
Over the last 100 years the S&P 500 took three long, interesting trips to nowhere,
underperforming risk-free Treasury bills for 53 of those 100 years (1929-1945, 1959-1982, and 1995-2009).

Are we setting up for another one of those periods where Treasuries outpace equities?
(All these episodes began with stocks selling for 20X earnings or better, much like today.)
I asked my Lucky 8 Ball for the answer and it said, 'Most Likley.'
So there you have it, make sure your seat backs and tray tables are in their full upright position!

I'll leave you with a chart I keep on my desk, it shows the inevitable oscillation
between greed and fear (overvaluation and undervaluation) that we investors have been continually replaying ever since Lucy stood upright'

upload.wikimedia.org - Stages of a bubble

Where do you think we are currently? Enthusiasm, Denial, Bull Trap?
Stay tuned for the answer!

All The Best!