I've been using Smush.it since it launched. Sure, its handy to have images smushed on upload but this assumes that people are using the WordPress media library. I can't stand all the image rubbish the media library throws into the database & don't need WordPress messing with my image sizes, so I don't use it. I've got a routine now - create images, send to Smush.it online, let Smush.it optimise the images & zip them, then FTP the zip file up, unzip on the server and its done. Quick, easy, and no reliance on any plugins. Smush.it won't break when WordPress updates either ;)