So are iPhones addictive, and that's what keeps us coming back for more? Even if the call quality isn't great, the camera doesn't work, and your screen's a bit broken (after only a year or so of having it), you still want another one. It's almost like a drug.
Apple has the most astonishing iPhone retention rate of 89 percent, and 4 percent of the remaining hadn't yet decided what their next phone would be, so potentially the actual retention rate could be a lot more. Now that's sticky!
Loyalty is very much key to Apple's future success. Sure they might be able to pick up the odd percent or two of growth from the failing BlackBerry brand, but Android's rate of growth is increasing at an astonishing pace and sales of flagship Android devices are overtaking new sales of iPhones in many areas of the world. So, with newbies to the smartphone scene seemingly opting for Android, keeping that brand loyalty and addiction to their products up there is hugely important to Apple.
There are other points here that have been overlooked too, besides the desirability of the products and the uplifting, almost religious, satisfaction you get from using them. Apple is more than a brand to its customers, it's almost a statement of who they are. iPhones are seen as being the best of the best, therefore using them must mean that you are the best of the best. You are a part of something, a movement, rather than just buying a rather nice product.
I should also point out that Apple's customer service is second to none of any other organization I've encountered, so keeping you happy once you have an iDevice in hand is truly important to them, which could be another reason for the high retention rate. You definitely get the feeling you are valued as a customer, rather than just being A N Other who bought just one of the millions of iPhones out there.