First, it was far easier to avoid overtime because I was already working 9 or 10 hour days before anyways. It was just how the tasks worked out. Coming in early allowed me to help get the team spun up early in the morning, and staying a little late allowed me to help the team clear any roadblocks so they could end their day too. Obviously it's more complicated than that, but that's a decent high level abstraction.
Second, it allowed me to cut Mondays out of my life. According to my Automatic Labs sensor, my worst commuting days are Mondays with easily 25-30% longer drives each way. Additionally, Mondays are horrible as there's a higher likelihood of hangovers with colleagues, or residual homelife drama that absolutely must be described between 7 and 10am. All the worst meetings happen on Mondays. It just goes and goes.
But after only a couple weeks I started to realize some other benefits. I started walking my son to kindergarten in the morning, and picking up my daughter from school in the afternoons, which allowed me many more focused hours of "play time" with them. I got several hours in the middle of the day (like right now) to read the web at my leisure, work on personal creative projects that had nothing to do with work, take some MOOC courses, etc.
Now, the negatives.
First, my wife who works part time 5 days a week imagines I sit at home drinking beer and watching TV at 9am. It took a while to really establish that just because I work 4 days a week doesn't mean I'm not still working 40 hours a week. I also do end up "working" on Mondays anyways, but just on education, home renovations, even if I do spend the first couple hours on HN articles.
Second, I had to institute a full communications embargo with colleagues on Mondays. It was just too easy for conversations about BS to slip back into work talk because that's how we roll in a normal workday anyways, mixing business and personal anecdotes. This could be hard for others to do.
Third, I find it nearly impossible to do any personal projects during the week now. Before, after an 8 or 9 hour day, I could come home and find some motivation for an hour or two of other stuff. Now, after a solid 10 hour day, I come home, decompress for about 30 mins, then it's time to start the bedtime ritual for the kids. There's no space or energy left for projects.
All in all, it was a great change. I've seen others who have completely flexible schedules (40 hours in a week however they want), or simply "five nines" where they get a day off every second week, and other variations. The moral of the story is: fuck Mondays.
0: https://automatic.com (not affiliated, just a customer)