The AAII (American Association of Individual Investors) maintains a sentiment survey with its members, and provides historical data all the way back to 1987. The graph below shows the delta between the % of people saying they are bullish and the % saying they are bearish.
There are some interesting points, such as a peak in bullishness in Jan-2000, or a record low in bullishness in Mar-2009, both of which were great points to exit/enter the market respectively. Oct-1990 was also quite low and another good time to enter the market.
However, there are also a lot of signals in the middle of the graph that don't seem super-correlated. Dec-2003 is a peak, but was a good time to invest, for example. If you look at forward-5yr-nominal-returns, the bullish % correlation value ("r-squared") is -0.32, and the bearish % is 0.13. As you'd expect, the bullish one is negatively correlated with returns and the bearish one positively correlated, but neither is very strong.
In general, I would conclude that only when the values are very significantly different is it worth considering them, and even then probably in the context of other more fundamental indicators.