I have the first four, I believe. I used the first with my son, S, when he was just on the verge of starting to read on his own. He was getting really frustrated with trying to read books that were a bit too difficult for him; he tends to stop doing something if he has any feeling at all that he can't do it now. I found they were good for him because they started off at a level that he had just passed, and they slowly progressed, so he never felt out of his depth. We would go over the letter sounds and words given at the start of the lesson, and then he would read to me. He usually wanted to do 2 or 3 lessons at a time.
I think he read all of the first one and part of the second (which is the first reader, IIRC, and starts off easier than the first one, the primer, ends, if that makes sense). By the time he had read those, he was much more confident in his reading skills and was fine reading harder books, but wanted ones that he was interested in, instead of the other McG readers.
He also likes the print and script alphabet in the primer, and working at reading the script sentences in some of the lessons.