This plugin does exactly what it's supposed to do, and it does it very well. Just a couple of easy clicks, and you're on your way to having all your thumbnails resized, which is often a necessity if you switch themes. However, if you have a decent sized site with lots of images (we have an image on every post, for thousands of posts), this can really do a number on your server. If you're using shared hosting, this could very well get your site temporarily shut down by your hosting company, simply because you're using up all the resources. That's not to say this is a terrible plugin, or even that they can do anything to prevent that. Let's face it, you are editing every single image on your site. There's no way to do that without beating up your server just a little. But it is something to be aware of going in.