My Take
I find Quill to be well-designed, particularly from the student-user's experience. The practice activities that are suggested by the application--based on diagnostic activities-- seem to sit within acceptable zone of proximal development, if perhaps a little on the 'unsupported' side of the spectrum. The teacher's UX is a little unintuitive-- for example, the way the various controls are linked via various tabs and sub-tabs, but I'm getting the hang of it.
How I Use It
I use this application with my Gr 7-8 class for both teacher-led lessons and independent, guided assessment and practice.