I was very reluctant to try invisible thread, but I was at a quilting retreat and was going to abandon my project as I was going to need many colors of thread to get the effect I wanted, and didn't have them because I was at a retreat. One of the ladies there said "Try this." It was invisible thread. I reluctantly tried it, and it was wonderful. On that occasion I was doing top stitching and left normal thread in the bobbin.
Since that time, I have used invisible thread in the bobbin for FMQ, and I love it. It doesn't seem to show the mistakes on the back the same, and so much fits on the bobbin that you don't have to continually be winding bobbins.
I have never tried invisible thread top and bottom. I'll be interested to see what others say.
amh