From the July 2007 issue of Psychosomatic Medicine, this:
Studies consistently show that mindful traits [...] and mindfulness meditation practices [...] reduce negative affect, stress, mood disturbance, and disease specific health symptoms across many patient populations
The article shows how mindfulness, combined with 'affect labeling' (or naming the feelings that you are mindful of) actually works in the brain. Turns out that there is more activity in the thinking part of your brain (the prefrontal cortex) and less in the fear part of your brain (the amygdala). The theory is that the thinking part of your brain "may disrupt or inhibit automatic affective responses, reducing their intensity and duration." In other words, you can better regulate your emotional responses. But there is more, as mindfulness is also associated with a reduction in symptoms for health problems like pain.
Not bad for a treatment that has no side effects...