This program only uses standard adb commands (Google approved) and isn't doing anything that will hurt your phone. It's also built using standard Windows tools and controls, and using the standard Windows api, so it isn't doing anything that will hurt your computer. It has been virus checked by Microsoft Security Essentials, with the latest definitions as of 04/11/2011.
That being said, if you use the Sideload Wonder Machine, it's at your own risk. Nobody is forcing it on you, and nobody is responsible for anything that happens. Especially me.