Baer (2003) is, I think, the most highly cited survey of the theoretical & empirical evidence (not cited by the WP article), and has a good historical overview of research. The conclusion states that effects are comparable for depressive patients to cognitive behavioural therapy, whose effectiveness as a therapy for depression is well-established (and incidentally, much more effective than Prozac). The effectiveness for relief of distress from chronic pain is also attested.

Kabat-Zinn, cited by Baer, is probably the most accessible scientist writing for a popular audience on this.

---------- Reference

Ruth Baer (2003) Mindfulness training as a clinical intervention: A conceptual and empirical review http://www.wisebrain.org/papers/MindfulnessPsyTx.pdf