I adore this app. Makes image compression a breeze. Is highly configurable without being complicated, and produces astonishing results if you are patient and willing to experiment to find the right compromise between compression time and file size reduction for your purposes.
Fast, reliable, easy to use. I would give it 5 stars if it didn't automatically overwrite the source files. I typically have to duplicate my files before running them through ImageOptim just in case the results aren't to my liking.
