We've been running four 10's (8AM-6:30PM) for a couple years now, and I don't think I'd ever go back. It took me a while to grow a big enough pair to give it a shot, but I'm sure glad I did. I don't think its for every business, but for any manufacturing type industry its perfect.
We started with Mondays as the extra day off, but ended up switching it to Fridays, which gives us the extra day if we need it for deadlines (we never do). Our customers have been completely understanding and I'll usually scan my emails on Fridays just in case something that needs immediate attention pops up (it rarely does). We're an appointment only shop, so we don't miss any walk ins.
It keeps my employees happy as well, as it allows them plenty of time to pursue other interests while still holding down a full time job, as well as take care of errands which can only be done during the week. Not to mention saving money on gas by commuting one less day per week.
If there's a downside to it, I've yet to find it.
