I went to 80% time, same benefits situation - keep insurance, prorated vacation and salary. Haven't worked a Friday in over a year. Don't honestly miss the money. It's a reduction but not 20%, because taxes come out of that lost income at your marginal rate. So it's a pretty effective way to buy time, really. And while it pushes out FIRE on the calendar, the number of days actually worked in that span decreases. Thanks compounding.
If you're doing it just to have time for other things and not because you're dissatisfied, stressed, or burned out from work, just go for it, don't look back, you'll love it as long as you currently like weekends. If you are one of the above (like I was when I made the switch - highly stressed, likely burned out), I recommend it as well, but know it isn't a cure all. I found that it helps immensely on days off, there's enough time to fully disengage and actually get out of work mode and it gets easier to do so more quickly as time goes on. I'm less stressed on average, but work isn't magically more enjoyable just because there's less of it.