I have used Sequential Spelling. It is best for children who read at a beginning 4th grade level or better. Some homeschoolers use it with younger children or lower-level readers, splitting up each lesson into two parts. My personal take is that it can be frustrating if the reading level is not yet beginning 4th.
In my opinion, the program works best for older children who are very poor spellers, as it seems to teach visual subskills other spelling programs take for granted. It would not be a good program for "natural" spellers because the amount of repetition would be too boring.
Nancy