It's been great, I have one day per week when I'm the only parent and usually it means a combination of housework (meh, but has to be done) and fun quality time with the kids.
I was actually a contractor at the time, and I think I was relatively lucky the client was understanding about it. After I stopped contracting I took a perm position and I was their first employee not working 40 hours/week. Again they were great, and it worked out fine.
I'm now Scrum Master there which occasionally can be an issue when I'm not there on Mondays, as one of my responsibilities is removing impediments for the team. But overall it works pretty well. I do feel like there's not enough hours in the week, but the most important things get done.
I wouldn't have done it differently. I'm wondering how things will change and what I'll do once our youngest kids are all at school.