I'm lucky to have been able to experiment a bit with this as my work is flexible and I'm in a pretty good bargaining position. I also do a fair bit of unpaid work out of hours.
Having either Friday or Monday I've found to be little different. Having alternating Fridays and Mondays is pretty awesome, 4 days on, 2 off, 4 on, 4 off.
There's a lot to be said for a mid week day off too. I'm looking at moving to a new employer and hoping to go negotiate a 3 day week, maybe Mon-Tues-Thur for example which will be heaven if I can pull it off.