I negotiated this with previous employers. I do consulting work now and I've negotiated this with myself as well :)

I think I'm more productive this way, because, after a 3-day weekend I generally have a fresh perspective and feel like I'm looking at the same tasks with new enthusiasm. When I worked 5 day weeks, even interesting tasks became drudgery and I'd stare at the clock continuously even when I wasn't working a strict 'clock-in' style job.