I work for a very large high technology company that has instituted a flex-time policy for about 10,000 people in California and Colorado. An employee may work traditional, 9/80, or 4/10 (4 ten hour days/week). Almost everyone from interns to senior management is on 9/80. The office is a ghost town on those "off-Fridays." I barely notice the extra hour per day and still have plenty of time for life outside of work. The three day weekends are priceless, whether you want to shop at the mall on an uncrowded day or go on a 3-day ski weekend. Even on the off-Fridays we're asked to work, management usually offers hour-for-hour vacation time reimbursement.
The downside of the 9/80 (besides the resentment of my friends at other tech companies) is that you often waste your off-Friday waiting for your wife, buddies, or whoever to get off work before you can leave for weekend trips. I often lament that the only other people with off-Fridays are my co-workers.