Are people who left academia happy with their decision?
I believe some of the national longitudinal surveys would cover this for the US. 

For instance, the National Longitudinal Survey of Youth 1979 cohort started tracking "middle school" age students in 1979 and is still tracking them. They have hundreds to thousands of attributes including education and many measures of health and happiness (though not all are updated every year). 

Another example is the National Longitudinal Study of Adolescent to Adult Health (Add Health).