I just tried imagemagick on my backup server.
Looks nice but it doesn't quite tell you how much filesize savings you got from compressing using imagemagick.
Whereas Smush.it does.
Also what is the difference between imagemagick and yahoo smush compression methods ?
I like how your plugin has an option to regenerate the images and run them through using your image compressor, but i think it would be nicer using the yahoo smush api instead for image compression seeing as that works well, and has a before and after value so you know how much space you saved.
Just a suggestion.