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Apple has released its biggest mobile update yet, iOS 5, which offers a slew of compelling new features and will likely make many iPhone 4 owners think twice about upgrading to the iPhone 4S.

As I pointed out in our roundup of iPhone 4S reviews, iOS 5 compatible devices will receive most of the software upgrades featured in the iPhone 4S, and that may just be enough for most iPhone 4 owners.

Sure, you won’t get access to the Siri virtual assistant, and you won’t get the upgraded 8 megapixel camera and faster A5 CPU, but you’ll have access to the new iMessage app, improved notifications, Reminders, and more. Altogether, Apple says there are over 200 new features in iOS 5.

iOS 5 is compatible with the iPhone 4, iPhone 3GS, both the iPad 1 and 2, as well as the third- and fourth-generation iPod Touch.

I’ve been testing out iOS 5 on my iPhone 4 for some time now, and I’m shocked at just how much better the experience is compared to iOS 4. iOS 5 basically makes the iPhone 4 feel like an entirely new phone.

The new notifications system means you’ll never have another annoying pop-up box disrupt your workflow. Instead, notifications appear elegantly at the top of the screen. They’re collected in a notifications tray that you can swipe down to access (yes, just like Android). Apple’s new iMessage app will definitely serve as iOS’s alternative to RIM’s popular BlackBerry Messenger. It’s faster and smarter than texting, and it also doesn’t require any extra texting charges, since messages go over the data network.

Dealing with the iPhone’s camera is also less cumbersome in iOS 5. You can now access the camera app by double-clicking on the home button, even from the iPhone’s standby mode. New editing features also let you touch up photos easily, including cropping and red-eye removal, instead of relying on third-party apps.

To access the update, you’ll first need to upgrade to iTunes 10.5, which was released yesterday. Users of older iPhone 3G and second-gen iPod Touch devices that aren’t iOS 5 compatible will have to be content with iOS 4.

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