Clinical psych graduate student and researcher studying, among other things, group mindfulness interventions in a clinical setting. Thought I'd weigh in.

While there is a lot of not so good research surrounding mindfulness, there is some decent research as well, some of which is mentioned in the original article. Mindfulness-based interventions like Mindfulness Based Stress Reduction (MBSR) and Mindfulness Basic Cognitive Therapy (MBCT) do show, as stated, at least moderate effects for stress, anxiety and general well-being. The difficulty that I'm finding in my own research (mostly still unpublished) is that these groups don't do much more than just being in any group would, and actually my current work is on parsing group vs meditation-specific effects (outcome variance decomposition). Meditation-specific effects are, indeed, proving elusive.

But there are a couple of things at play here. The first is that, it's nearly impossible to separate the effects of ANY short-term treatment from so-called "group" or "therapist" effects. One of the major findings of the last 10-15 years is that a therapist or group-leader's ability to convey confidence, warmth and understanding and to establish rapport is a massively more important factor than technique, at least in the short-term. We're talking 80+ of treatment gains. Of course, to do that you generally have to have at least an effective-seeming technique, and one that you legitimately believe in, but it doesn't seem to matter which one. Again, this is in the short-term.

The second issue, is that virtually all studies in any clinical intervention are really quite short. MBSR is 8 sessions. How good do you think a person can really get at meditation in 20 minutes per day for 50ish days? The answer is, not very. And that's true for other cognitive and behavioral treatments as well. Studies limit the length of their tested interventions to between 8 and 20 weeks, and usually on the lower end, for funding and convenience reasons. Then they (we) find that group and therapist factors trump technique. It's really not that surprising when you think about it. It's not that the techniques don't work, it's just that it takes as long to master your mind as it does to master anything else, 100s to 1000s of hours. And that research, which would take years and be extremely expensive, just isn't done.

So yes, a lot of the mindfulness work out there is hype. But it doesn't really bear on whether mindfulness is effective, just whether a person can make massive gains in 2-4 months. My own view, is that mindfulness is an effective tool for teaching awareness and attention control. I use it in conjunction with other cognitive and behavioral techniques with my own clients.

Review paper on mindfulness effect sizes for anxiety and depression. I've seen others which found moderate rather than large treatment gains:

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2848393/

The "Dodo Hypothesis" that the bulk of therapeutic treatment gains are the result of "common factors" (therapist effects, rapport, empathy, etc.)

http://onlinelibrary.wiley.com/doi/10.1002/wps.20238/full

I think the second one is behind a paywall, but you should be able to grab the DOI and "find it".