You may have read on the internet how great this plugin is, which is true for the concept for the plugin.
And yes it does work, however it has limitations that few bother to mention.
1. It cannot bulk smush images
2. It cannot indicate to you whether it is smushing the image or not. I had to manually check by saving the image back to my desktop and comparing the original and the wordpress uploaded image to confirm there was a file size difference.
3. It cannot smush images that already exist inside your wordpress site before installing smush it. However someone did point out that there is a plugin called AJAX Thumbnail Rebuild which will rebuild your existing thumbnails to go through smush it. But that solution carries problems of its own.
4. Smush.it, service that is used for images' processing, has limit of 1MB per file so you can't do anything about it.
Anyway, when i say i get no indication if smush.it is doing anything, is because when you upload an image, there is no indication or message saying it's smushing, or whether it managed to reduce the file size or anything. So i manually had to check myself to confirm that it worked, which it did. But .... when i checked on another image which was 1.3 mb in size, there was no file size reduction. I tested smushing it on the yahoo website, and it managed to reduce the size. Then why wouldn't it reduce the filesize using smush.it ? This is why it is important that the plugin has a clear indicator whether the smush.it plugin managed to reduce file-size or whether it could smush the image to begin with.
Was told that smush.it plugin doesn't work for 1mb + file sizes, which i confirmed with the wp-smush.it plugin. But when smushing using the yahoo site using the same image, it managed to reduce the file size, why is that ?
In assessment, although it would be great using the wp-smush.it plugin to auto smush files; it gives no clear indication if it is smushing the image or not, unless you trust/believe that anything below 1mb will be smushed without having to re-check to confirm that.
Also the plugin does not allow smushing previously already uploaded images.
So my conclusion is, that until the plugin adds in a clear indicator like whether it managed to smush, a file size % reduction indicator; then i rather just use the yahoo website to do my smushing.
It has the potential to be a top10 must have plugin for wordpress, but it needs to be fixed up to make it so.
Like i said, at the moment i can't justify using this plugin over the yahoo smush site for now.
However i must give due credit to Alex Dunae for trying at least to make a useful plugin during his free time.
I hope you don't take this the wrong way as me being insulting or anything. Just giving my honest feedback so the plugin can be improved.