I switched to 3 days weeks almost 2 years ago. (Mostly, I remain in a company chatroom on other days for people to ask me questions, and if there's a massive disaster I'll help if I'm available).

What I've found is that I spend the 4 days I'm not "working" organizing my thoughts and plans as a background task and this means most of my 3 days is spent executing. Whereas at my previous job I would spend several days a week on thinking and organizing and another couple on being blocked or bored.