I've used the 5-7, 6-8 and now 7-9, but never exclusively. There have been things I have loved about it, and other parts that I think are a waste of time. We use OM as well, and I am very happy with the mix of the two.
It seems like there are lots of "pages" for a program that says it moves off of them. I understand that the idea is often to offer alternative levels/versions of the same lesson to choose what you think is best for your child but even if you take away those extras it's still a thick book, imo.
I love teaching/learning through literature. I have had good success finding most books through the library but have found many that we really enjoy and want to purchase. You definitely do not need to own every book- many are only referenced a few times- but unfortunately until you actually see the book and look carefully at the lessons you won't know ahead of time.
Beginning with the 7-9 there are literature units which go along with the concept. I love this! They are good titles for the most part and offer good activities ( or a jumping off place for me to do more) and ideas for in- depth thought and conversations. There is enough in there to keep you going and feel like you are covering some interesting topics, imo.
The program doesn't "wow" me. There are annoying typos and in the younger levels the printing on the pages is messy and unprofessional. We use the idea of main lesson books to keep the units together, and are starting lapbooking for the literature units which is fun and interesting for my son.
There is a yahoo group and lots of turnover so you are able to buy most items used, sometimes even with some trade books included. I've never paid much more than half of the retail price. Hth's...