Review: Apple iPhone 4S for AT&T
iOS5 makes some noticeable improvements to the iPhone 4S's messaging capabilities. First and foremost is the iMessage system.
iMessage uses Apple's push servers to send IM/SMS-like messages between iOS devices. This means iOS 5 users — be they on an iPhone, iPad, or iPod — can send IM/SMS messages back and forth without incurring texting or other fees. Don't have a phone number associated with your device? No worries, you can use an email address instead (for iPad/iPod Touch owners, mostly).
iMessage offers read receipts, works quickly and reliably, and is a nice alternative to using carrier-based texting services, but it has limitations in that it only works with other iOS 5 devices. The iMessage app is the same basic application that's used for sending text/video and other multimedia messages. You can do all of this within iMessage just as with the normal texting service. The one really neat thing is that iMessage automatically tells you which of your contacts are iOS 5 users so you can send them iMessages instead of text messages.
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Beyond iMessage, the email application adds a few ho-hum features, such as support for "starring" emails, and such, but the basic functionality and features set is about the same as with iOS 4. Mail now supports search within the email body, inbox add/delete, and more text formatting, such as bold, italics, etc.
The native Facebook and Twitter applications are both excellent, but you have to download them yourself, they are not included. They offer full access to both services, and function very, very well. Twitter, in particular, has also been baked somewhat into the operating system itself. For example, let's say you're browsing through your photos. In iOS 4, if you wanted to Tweet a photo, you had to open Twitter first, then use the Twitter app's tools to find the photo and send it. With iOS 5 on the iPhone 4S, this feature is built directly into the photo gallery application. Ditto for the browser and other apps.
Facetime is still available and works identically to the version in iOS 4. I find it to be among the easiest video chatting applications to use, but it is limited to chats with other iOS device users (and Mac users) on Wi-Fi only.
If you're interested in AIM, Windows Live, or Yahoo instant messaging, you'll have to go fish in the ocean that is the iPhone App Store.