Is it a good time to invest in S&P500?

Depending on your time frame. If you are a long term investor, it is always a good time to start investing. Because as time passes, the timing risk of investing will diminish.

'Invest', probably No.

'Trade', yes. Fundamentals are not reflected in the direction of the markets. The rally in the last 3 months are attributed to QE4, another unprecedented liquidity injection by the Feds. There are already several ticking time bombs brewing. But as long Fed keeps the music going, the show will likely continue for sometime.