If someone gave me the chance of a 30-hour work week, I would rather work 4 days of 7,5 hours than 5 days of 6 hours. Just the hassle of getting up early, dressing up for work, commuting etc. isn't really worth it when I'm only going to work 6 hours anyway. I'd rather take an extra day off.
Usually it's a split in the middle of the week or something, for example my manager doesn't work tuesday's, it's his "papadag" as he then spends time with his kids and don't have to send them off to day-care or get a nanny.
Another colleague of mine dropped his contract from 40 hours a week down to 32 hours so he would have an extra day to work on his music career and an upcoming album.
Obviously these arrangements will affect your salary as you work less.