iOS 5 has a lot of cool features, and TidBITS has a great article walking you through some of the useful features that you may have missed. Some are for both the iPhone and iPad, while others are just for one device or the other. Regarding features common to both devices, the TidBITS article covers the new Shortcuts feature, the ability to sync wirelessly without being plugged into a power outlet, using Airplay to mirror the screen of your device on an HDTV, and an explanation of how to change the order of apps in Notification Center.
For the iPhone 4S, the atticle notes the new week view in the calendar, location-based reminders, opening the Camera app quickly when the screen is locked, and the option of invoking Siri without using the Home button. For the iPad 2, the article covers the new split keyboard feature and how to use gestures to invoke it, the new mailboxes view, and the ability to switch between apps using gestures.
The Shortcuts feature is pretty neat. If you have a long word or phrase that you type often, you can create a shortcut for that word or phrase. Typing the shortcut will insert the word or phrase into your text.