You can also expect dock peripherals of all sorts -- from charging stands to portable stereos, from car audio systems to camera and recording gear -- to not work with the adapter. There's no easy way to know until you try. All I can suggest is that you get a fifth-generation iPod Touch and a Lightning adapter and see how that works first; return them if they don't and skip the iPhone 5 until native peripherals are available. For the likes of a car stereo, that may be never, so either switch to Bluetooth audio streaming or stick with an iPhone 4S or earlier iPhone.
The switch to the space-efficient Lightning connector poses the biggest loss to iPhone users, but beware of other gotchas.
For example, the iPhone 5's deeper screen (1,136 pixels) is unknown to most apps, so they won't extend to take advantage of it. Instead, you'll see a border where the old screen ended (960 pixels), though the width remains at 640 pixels. Developers can update their apps to sense the device and change the screen geometry accordingly. Apple says it will make that update for its apps by Friday, but that could take weeks or more. In the interim, you'll have an experience gap across your app library. That fractured user experience is very un-Apple.
If you're an international traveler, also be aware that the iPhone 5 uses the new Nano SIM standard, and your existing SIM cards won't work in it. Carriers have yet to produce the new Nano SIMs; until they do, you won't be able to swap SIMs to use the iPhone 5 in other countries.
Finally, there's been a minor firestorm of complaints in social media and the blogosphere noting that the CDMA versions of the iPhone 5 for Verizon and Sprint don't allow simultaneous use of voice and data. That's true, but it's been the case in the two previous CDMA iPhones (the 4 and 4S). At its core is how the carriers deployed CDMA, dedicating the whole LTE channel for either voice or data, not splitting it. That allows for better speeds, but limits flexibility. That's why if you use the Find My Friends feature in iOS to track an iPhone user, he or she disappears from monitoring while talking on the phone.
It's not an Apple design flaw, but it does reflect Apple's choice not to use the workaround employed by many Android devices: They switch to a 3G channel for voice while LTE data is active, in essence using two simultaneous connections. That also means they need two active radios, but Apple has included just one in the iPhone.
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