My company (Administrate - http://www.getadministrate.com) went to a 4 day, 32 hour week (but paid for 5) a little over a year ago. It's been a huge success, and we haven't seen any drop in productivity. We have had to be a little bit more efficient in some areas but overall the productivity gain was more or less "automatic". I'd recommend trying it to anyone, but understand it can be a scary thing to implement, mainly b/c it can be an easy thing to blame when things aren't going well. We have had some brief periods (usually when we're hiring) where teams have had to work 5 days, but overall we've been pretty consistent.