QQQ is an ETF of good quality US stocks. 

It has achieved remarkable growth and gave an excellent return over the past many years. 

However I will not buy this ETF. Why?

It is far too high. It has a price earning ratio of 49.7 times and a dividend yield of 0.77%.

The price is 2 times of fair value. If it dropped by half, the PE ratio will fall to a more normal 25 times (which is still high) and the dividend yield would increase to 1.5% (which is not great).

If QQQ is not worth buying, so are most of the US blue chip shares.