Yeah. I worked a 3-day week for 6-7 years, and overall it worked really well for both my employer (who got a more productive employee) and me.

From my side, the biggest drawback was that it was a sort of golden handcuffs - I almost certainly stayed there longer than I otherwise would have because getting an equivalent setup elsewhere would have such a pain.

My impression was that the biggest drawback on my employer's side (after hashing out the initial bureaucracy/paperwork) was my more limited availability for meetings.