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Not a lot of the apps for the iPhone have focused on providing something visually stunning over usefulness/enjoyment. Earth3D is one of those apps. You won't be learning anything new from Earth3D or really getting anything useful out of it, but you'll be able to see a beautifully done 3D model of the earth in full rotation.
The app doesn't allow you to do much either. You can view the Earth rotate on its axis with the sun and moon orbiting around it. In this mode, you can actually stop, reverse, or speed up the rotation of the Earth by moving it with your fingers. But if you double tap the earth, it stops moving and allows you to look at it straight on, rotating it on it's axis at your own pace.
You can't look at the Earth in a free form mode, making it awkward when you get to see the moon circle in front of you but not the sun due to how far away it is from the Earth when in rotation. You'll only see the sun peak out from corners no matter what mode your in, giving you a feeling that you aren't really in control of anything regarding this app.
If you want to show off just how beautiful of a creation the iPhone is capable of displaying, then this is a great app to do so. But the fact is, you won't have any real use for it other than that. The app doesn't freeze at all and provides you with plenty of great visuals for you to enjoy, but it's really the only thing of its kinda at this point. Hopefully we'll see these graphics used elsewhere.