For me, it's not even as much about performance as it is about coverage. I've said this in a million threads on the topic, but T-mo and Sprint just don't exist here. I think I'd be in about the same boat as the author of that article with how much I could save each month, but I'd be running at what the Project Fi page calls "2G". No thanks.
I had T-mo when I lived in Kansas and loved it, so if I was still in that area, I think Fi would really make sense for me, but I'm pretty well stuck in this area now, so unless until one or both of the carriers it relies on add a ton of towers up here (I've heard rumors that the FCC won't allow it, though T-mo's projected plan for 2016 shows most of the UP covered...?), it's just not viable for me to even give it a try.