I went part time (60%) for about a year and a half at Red Hat. For context: I'm pretty senior (architect for the Gluster project), had been there a while, and the clear alternative was that I'd quit. I only mention that because I think I wouldn't have been able to pull it off otherwise.

First off, my boss at the time made a very wise suggestion to start with a month completely off. Not even checking email. My first thought was that it was to affect my own behavior, but the most important part was to change other people's habits. They couldn't count on me to drive anything, resolve anything, fix anything, for that month. They were forced to learn how to fill all those gaps themselves, so when I came back part time I wasn't immediately deluged with a full-time level of demands/requests (or worse because of the backlog). Great idea.

For the first year, it went pretty well. I don't feel like I benefited so much from the reduced hours as the increased flexibility. I could work when I wanted, fitting work around my domestic responsibilities or the ebb and flow of my own energy levels, instead of having to fit everything else around work. It's amazing what a difference that makes. Also, I could take a three- or four-day weekend whenever I wanted, without having to take any PTO. I'd just front-load the week before and back-load the week after to make it fit. Very nice.

Over time, I did start to work more than my nominal part-time hours - as I'd already expected. Mostly that's because I was feeling recovered from the burnout that had triggered the whole process. So eventually I talked to my (by then different) boss and went back to full time. Then the cycle kind of began again and now I'm not there, but that's kind of another story. It really did help, and I highly recommend that anyone who has been running flat out for 15+ years (for me it was over 20) try part time to see if it suits them.