One of the new features in iOS 5 that I'm using all the time, but didn't expect to, is the new Twitter integration into Photos, Safari, YouTube and Maps.
Setting up your Twitter account is pretty easy. Go into your iDevice's settings and log into Twitter. Multiple accounts are supported. When you take a photo, you get an extra option to tweet it from the Photos app. Clicking the tweet button brings up a new tweet with a thumbnail of the image you're about to send. Type in your message, add your location if you want to do so, then send. Your phone will chirp at you once the tweet is successfully sent.
If you have a contact's Twitter handle in your address book, then when you do an @reply it'll bring up the friend if it recognizes their Twitter username. If you have a large list and can't remember everyone's Twitter names, it's a good idea to plug them into your Contacts if you plan to use this feature frequently.
Photos isn't the only app you can send tweets from. You can also do so from Safari, YouTube and Maps. The most useful of these by far is Photos. There's a lot to be said about the basic Photos app in iOS, and I toggle between that and Camera+ when taking images. I'm happy to be able to tweet from both now, and I actually find that the built-in Twitter integration is the one I'll go to first unless my image needs adjusting in Camera+. It'll also be useful for taking and sharing screenshots straight from Photos.