There are a couple of things that cause a lot of pain.
One of those is the backup not being from the same iOS version. It will let you restore it to a new device, but then it doesn't complete (ie apps and such).
Just went through this going from an iOS 5 iPod Touch and an iOS 6 iPhone 5. The downside is that it makes old devices like the iPad 1 (which can't run iOS 6) going to say iPhone 5 (which can't run iOS 5), impossible to restore.
One of those is the backup not being from the same iOS version. It will let you restore it to a new device, but then it doesn't complete (ie apps and such).
Just went through this going from an iOS 5 iPod Touch and an iOS 6 iPhone 5. The downside is that it makes old devices like the iPad 1 (which can't run iOS 6) going to say iPhone 5 (which can't run iOS 5), impossible to restore.