Kraken is a web service designed to perform a similar function to desktop based applications such as ImageOptim. For as little as $7 a month (for half-a-gigabyte of images processed a month), you can have Kraken.io process your images and compress them. Alternatively, you can use their free web service by uploading files individually. The service works significantly faster than ImageOptim because of the powerful servers that they use to crunch your images.
But, how does it compare to the desktop equivalent?