I did the same. Asked my employer to work 4-day weeks to lower the pressure after the stressful period during which we released our first product.
Life satisfaction I get from that extra day off if quite huge, as I almost get twice as much free time compared to the time I actually work (3/4 vs 2/5). That time allowed me to get better at playing music and rock climbing and I now have time to read and learn more.
Productivity wise, I probably do a little bit less than I did while I worked full-time. But I'm more productive for every hour I work, which is not really a bad deal for my employer. We have several offices, and most people I've been working with remotely didn't notice that I was working part-time for months.
I also got a 20% pay cut, which reduced my after tax income by about 15% (living in Belgium). Lifestyle hasn't changed, but it definitively has an impact on how much money I can save every month. I'm considering buying an house, and working part-time will definitively delay this by a couple of years.
I agree with you that working part-time could seem career-limiting, but my opinion is that it's more like an inner feeling than something my supervisors think of me (it's a little bit awkward to work part-time when most of your colleges work full-time). In the end, the average gap in productivity between an engineer that works part-time compared to the same one that works full-time is way less than between two given engineers.