We tried this in a company I worked at previously. Productivity wasn't affected, but people worked harder during those shorter hours.
After the experiment, people went back to longer hours as they were hourly paid and they didn't want to lose their money. During the experiment we paid them the equivalent of an 8 or 10hr shift.
Shame they didn't implement it as it proved that productivity and efficiency was not affected, but the intensity of work was all that was missing.
In industries where a culture of "clock watching" goes on, it'll never work. In a more dynamic work force, or one, at least where younger directors are in place, I have found that flexible working to be as good at delivering results as not. This is anecdotal from my own experiences - but still, I think from the change management work I have done professionally I have seen enough evidence for me to think it should work.
What is key, is that cultural change to create that paradigm shift in behaviours. Until that happens it is too disruptive for most industries.