The SMH yesterday wrote up this report which, as many other analyses have, finds graduates are not happier than other people (though the research is mixed on this; some studies do find a benefit, and in the 2007 Australian Survey of Social Attitudes sample graduates are happier).
Education and happiness in the school-to-work transition by Curtin University’s Michael Dockery is especially interesting on the question of graduates and happiness because it uses the the Longitudinal Surveys of Australian Youth (LSAY), which tracks the same individuals over time. They start when the respondents are in Year 9 and finish when they are in their mid-20s. This lets us see happiness over time and the possible effects of changing circumstances.
Happiness relative to mean, by educational attainment

Source: Figure 2(b) in Education and happiness in the school-to-work transition, published by NCVER
What this shows is that people who will eventually get undergraduate degrees start out with above average happiness and end up with slightly below average happiness. People who will eventually get postgraduate degrees are the happiest in 1997, but only average in 2006. By contrast, those who destined for lower qualifications are relatively unhappy in 1997 but happier (relatively, and in asbolute terms) in 2006.