I'm regularly asked this questions by clients, friends, strangers (once they know what I do), and by myself. My answer is usually: "yes. ," but it almost never seems like a good time to invest. When it does, watch out.
I've been doing investment work for 30+ years and following the market for nearly fifty. There are always multiple reasons not to invest: wars, terrorism, economic instability, political gridlock, interest rates, gas prices, natural and man-enhanced disasters ...and finally, the stock market, itself.
As I write this today there is talk of the federal government defaulting on its debt obligations (U.S. government bonds), historically considered to the safest investment in the world. Gasoline prices (not adjusted for inflation) are nearing an all time high and gas-price hysteria has returned. High unemployment figures continue, despite a brightening economy. Japan is just starting to recover from a series of catastrophes. And, the Middle East is in turmoil, again.
If I picked ten random days from the last thirty years, I'd guess nine of those would be days when it would make no logical sense to invest--unless you planned to invest for the long-term, diversified yourself properly, kept a prudent cash reserve and avoided consumer debt. Today would be one of those nine days.