I'm using this curriculum with a young man who was left choking in the dust in the standard school curriculum. He's able to work this independently at his own pace. Now that he's finally found a form of math instruction he can understand, that pace is quite quick!
We are moving to Saxon from ABeka and took the Saxon assessment test. According to the assessment I was to use 5/4 for my 4th grader. We are currently skipping ALL lessons as we are in the process of taking each test until we find one that is too challenging for my daughter... if she doesn't get 100%, we will go through the lessons. We are currently on Test 6 and she's aced them all, so I would say the assessment test is not very accurate... which means my money is not well spent as I will not get to utilize the entire curriculum. Seems like a reasonable enough curriculum but I won't know until I actually get to use it!