The projects that worked best for me at 60% were those that involved lots of independent work. I'd find something big that the team needed doing, that was important but not urgent--the kind of stuff that always gets put off until too late. Then I'd go off for a few months and come back with a prototype for approval. When it got through review, I'd go off again and make it solid, write all the tests, develop a staging strategy to get the parts in, etc. Once it was in, I'd find another project. I did a couple of really big things that each took 10-18 months that way.
The minuses of part-time work are pretty obvious: you have fewer work hours per week, but the same amount of overhead; you're harder to reach, since you've effectively got longer weekends; people aren't really sure if you're committed to the project or just slacking off.
However there are also pluses. The biggest one is that you have more mulling-problems-over-in-the-shower hours per work hour, which can make your work hours a lot more productive. A similar one is that if you're ever waiting on back-and-forth with other folks for information, approval, code reviews, etc., you get more hours of those per work hour as well. Another is that you take far fewer vacation and sick days. When my usual schedule was M-W, if I or a child was sick on a Monday, I'd just work Thursday to make it up. And I never needed a vacation day to take a long weekend, get shopping done during working hours, go to the doctor, or anything like that.
Communication with your coworkers is key, so that they know what to expect from you. And be sure to work within your company's culture. If you'd answer an email on the weekend, then you should on your day off. But it goes both ways: they shouldn't regularly ask you to come in on a day off if they're not asking others to come in on the weekend.