My company does 9/80 work weeks and I find the experience mixed. I tend to have about 6 'good' hours a day for solid engineering design and after that the ideas don't flow as well. 9/80 works when you have lots of paperwork to do (perfect for a government contractor) but doesn't do so well when you have actual design work. Doing errands on weekdays or making it to the gym are impossible unless you have an under half-hour commute. On the other hand, the every second friday off is nice. In summary, it only works if you can you work steadily and don't mind cramming all your errands into the weekend.