Allan M.
Yelp
Visited during Doors Open Milwaukee 2016, and it was awesome to do a visit here. Didn't bother me that it was the most out of the way place I visited for sure during this year's DOM event(and barely before the last bus stop of the #57 Milwaukee County bus route), since I love visiting ANY Frank Lloyd Wright buildings. If you've ever been to Unity Temple in Oak Park, IL, this will feel a little more open inside vs. the sanctuary of Unity. They both are great structures in their own ways, though.
The pastors and other church people were very nice in explaining the history of this place, and I enjoyed seeing the decorations(i.e. vertical lights in the middle of the stairways, stained glass windows on the balcony) inside this church. Definitely a must visit, if you're in the Milwaukee area and are an architecture buff. Photos are definitely permitted inside this building, btw.
I have no idea what hours and days they're open to the public regularly, but I imagine if you called the church up, they'd probably tell you their public hours. I'd bet the church's building will be open for next year's Doors Open Milwaukee(September 16-17, 2017), and for other church fests as well.