I find even in the best jobs, there's a general level of stress that builds up at work over consecutive days. There's a great feeling being a bit stressed on Tuesday and thinking "ah it's ok I've got tomorrow off". It's like that "Friday Feeling" twice a week.
I preferred the large block of time off, I found that I enjoyed Friday-Sunday more knowing I had a bonus day on Monday to relax - I could really pack in stuff over the weekend without worrying about being exhausted back at work. And I found the Wednesday off just felt... wasted. Nobody else was free, nobody really wants to do things on Wednesday, and I wasn't tired enough to want a day of doing nothing.
Also, sometimes I need a few days momentum to get something over the line and two days is often not enough. Anyway, just my 2p, more to the point I would totally jump at the chance to have a 4 day week permanently.
Specifically, when I go to my GP, the staff are always busy, don't appear to get much of a break at all. I doubt a half day on a Wednesday makes up for extra hours put into their job just to get through the week.
The downside of the two day (Th and F) block when I wasn't around was that it was hard to contribute to projects that needed high levels of ad-hoc customer interaction. This actually meant I had to walk away from a couple of projects that were technically appealing, but I was at the point in my career (hence three-day week) where life-work balance was a higher priority, so this wasn't a hard decision.
Part of my decision on the timing was also influenced by discussion with women in the company who came back to a three day week after maternity leave. They pointed out the pros and cons of the various options and made it easier for me to pick the right option from the outset.