I always use them for all my sewing except quilting and machine embroidery. I have two: a generic plexiglass for downstairs; for upstairs, the one I use is also a clothsetter, designed for beneficial placement of extended machine embroidery designs (for which I have yet to use it).
For quilting, however, I find them awkward and a disadvantage. They are fine when quilting very small projects; for anything large, you need something that's not only the same height as your machine bed, but that also covers a large area both behind and to the left of your machine. I used to use a huge cardboard box backed up to the plexiglass extension, but I still managed to occasionally get my quilts hung up on the side and front edge of the plexiglass. I felt more like a quilt wrestler, rather than a quilter. I got the quilting done, but it was no fun.