There is one systematic review on this topic. This study was published in the American Journal of Clinical Nutrition, which is the most respected journal in their discipline. As so, I consider this study perhaps the most important publication on this topic. Main conclusion was that the literature is scarce with regard to intermittent fasting. Small studies have been published, and these studies have shown positive outcomes with intermittent fasting. It is evident that these findings must be replicated in a study population reaching 1000+ participants.
Some sources claims that currently they are conducting the largest study in intermittent fasting in Sweden, lead by professor Kerstin Brismar. Apparently the preliminary results are encouraging. No published studies are yet available. Following pages (in Swedish) have some more information: svt.se & Karolinska Institutet. Prof Brismar is a well-known researcher in metabolism and has a great credibility in her field. I am looking forward to their results.
There are hardly any harms with intermittent fasting, I think. But I think it has a great pit fall. One must have strong spirit and psyche, since the human nature has a tendency to give "rewards". In this case it means that after two fasting days one should not eat more than is normal (+no candy etc.), since this would ruin the meaning of the diet.