This plugin probably make the page load some faster. But it breaks the design and the harmony of the page content if u have images that you want to "build up" the site in a design way.
I have sliders and also woocommerce products and all those images just pops up in a way that is not intended throwing all texts and titles all over the place until the images finally is loaded.
I have optimized pages with small images so the time for loading the page is not really affected by this plugin for me. Maby for photo blogs and that kind of sites with big images its a realy great plugin.