One of the great things about Opacity is how it can simplify your workflow. Check out how much easier it is to make graphics:
Factories let you export images in any supported format with just one click. You can setup factories to upload, email or instant message an image to your collaborators (or clients).
With Previews, you can view your image as it will appear when deployed on your site or in your app.
You can also keep multiple resolutions for images and even multiple related images with frames in the same document. This lets you export all related images together.
Opacity also combines support for all of the formats you'll need for screen graphics in the same application. If you ever want to change formats, just pick what you want from a popup menu. Opacity will even warn you if there's any potential issues with saving your image in the format you chose.