The plugin works great in one respect - it doesn't completely destroy the image quality of an image on upload. There are two bugs, one of which is more serious than the other.
The first is that setting an image quality of 100% doesn't set the quality flag on the JPG compression, so it winds up using the ImageMagick default. Setting to 99% solves this one.
The second is more serious. The plugin fails to generate the various alternate image sizes when the checkboxes for it to do so are ticked. This has the side effect of causing the thumbnail editor not to load, since no thumbnail exists. Unchecking the Thumbnail and Medium boxes for the plugin solves this problem, which then leaves you with the artifact-laden GD scaling for those images.