The ISM is not only a solid indicator of future economic growth, it is also a reasonable gauge of stock market momentum.
At 52.1, the ISM index points to an annual increase in the S&P 500 Index of just below 5%. Currently, the S&P 500 Index is up exactly that, implying that macro and stock markets are perfectly aligned. Obviously, this relationship in itself is not perfect, as other factors such as trade and future monetary policy shape the path of the S&P 500 Index as well. But I believe a neutral stance on equities is warranted for now.