I've tried using this plugin on a client's site, but the plugin does too much and I've wasted a lot of time tracking down a problem that ended up being caused by WP Fluid Images in Internet Explorer 7.
The site I'm currently working with is http://beta.upicsolutions.org (may not have WP Fluid Images when you visit). I have three images on the front page that are created from a Page that is included via a Featured Page widget onto the front page.
When I remove the width and height from the <img>'s, WP Fluid Images hides the images by changing their width to 0% as soon as the script loads. If I set a width and height on the <img>'s, the WP Fluid Image shrinks the images down into icons.
Again, all of this only happens in IE7.
I wish WP Fluid Images had an admin interface where I could selectively disable some of its functions. Like I only want the width- and height-stripping from image uploads, I don't want the other automated script functions.