i haven't used this, but I wanted to share some information I recently read in case it helped your decision. They are discovering that early teaching actually hinders deeper development later on. It is actually not good to use early readers or early anything, unless your child is wanting to read/learn that thing and expressing a deep interest in it.
They've discovered that a young child's brain is not fully developed and when you teach them these things, they develop a less efficient pathway in the brain because they are under developed. Whereas, if you were to wait longer, the brain would develop more, and the child would then develop a much more efficient pathway in the brain - leading to deeper understanding and a deeper ability to continue learning.
Preschool and early learning are not showing in studies to be beneficial. It's actually showing opposite. Sure, some kids will be interested younger, and I think you should let the child's interest lead the way. But in the long run, it doesn't mean one child is smarter than another.
There's another study I read that showed one country starting school at 5 years vs another country starting school at age 7. The children at age 7 learned much faster, had better comprehension in the long run, and clearly did better waiting to be older to learn when compared to the 5 year olds.
Children are very good at memorizing, but it doesn't mean they comprehend and understand. So, I think for a while people were thinking children were truly learning these things when in reality it wasn't a deep understanding. Again, that's not to say that some children aren't truly understanding at a young age...maybe their brain has developed faster. But even in those cases, there's no harm in waiting longer.
Anyway, hopefully this is helpful info for you. I used to be very interested in early learning until I looked it up and realized how much better it is to wait. That being said, my son got interested in reading when he turned four. I went ahead and started teaching him. In a few month's time, he was up to 1st grade level reading. He was no longer interested after that, so we stopped. But it shows that when they are interested, they can just soak it up and do it fast! :-)
Good luck!