I chose 8 or up for this if kids will be reading this on their own. If parents will be reading to their kids, this would be great for younger kids too. One reason why I say this is because the grammar used is not always the best, and can even be confusing at times. An older child will probably understand the message despite the grammatical errors, and a parent will be able to relay the message to a younger child while reading out loud.
We have an iPad and the graphics haven't caught up with the latest graphic capabilities of the iPad, although they probably look very good on the iPhone. The "3D" isn't really impressive, and shouldn't really be touted as anything extra. But the pictures are clear and accurate, and your children will get a good idea of the appearance of whatever subject is being discussed.
We haven't downloaded any of the extra packets yet. I read a review about a man who paid for the extra packets and couldn't get them to download, but I hope that his experience is an isolated one.
Considering that 30 years ago we used to pay $300 for a set of encyclopedias that contained a set of static information with pictures of lower quality and a more difficult interface, I feel that a $2.99 price is a steal for this app.