A product is successful when it sells hundreds and thousands of units.

A product is insanely successful when it creates an industry around that product. iPod is one such product as there are companies (and there are divisions of large companies) that make products just around iPod.

A product is insanely-unbelievably-extraordinarily successful when companies try to design and roll-out products that are possibly used only by a small percentage of people.

Take a look at the two products that are in the Skymall catalog related to iPod

One of them is a case where you can secure the iPod and it can float along side with you in the swimming pool or wherever appropriate

Second one is an iPod jacket that is water resistant up to a few feet in the water. I think just in case you need to listen to the songs when you are swimming under water.

As you can see, these manufacturers thought that (what I think is a small percentage) the subset (people who will use iPod in swimming pools) of the entire iPod universe is BIG ENOUGH to go and create the adjacent products in that area.