I work at the US Environmental Protection Agency, and we have various flexible schedules open to us. I use the most common, the 9/80 with alternate Fridays off, and it works fine for me. It's very nice to be able to be home during the week for, say, a visit from the plumber, or to arrange for an appliance to be delivered and installed.
It's especially good because it's optional. Others in my office, especially those with small children or difficult commutes, don't necessarily appreciate such a long day and so don't use the option, but I live in DC proper and it takes me under 45 minutes to get to and from work on public transit.
An additional plus is that I can switch my day off on an ad hoc basis by advance request. It doesn't have to be a Friday, either; some of us have Monday off, and I know someone who chose Wednesday (why, I don't know).