I installed the plugin and Retina images are generated automatically, checking my WP Uploads folder I can see the x2 files in there and the Retina panel in the Media section also shows me that all files are generated.
When I use the server side method files are just displayed huge, since you point out that there could be caching issues with the server side method I switched to the client side method. With that the images are displayed in the correct size again but the original image version is used instead of the x2 version as you can see on my test site at http://www.mfephotography.com/gallery/nightlife/
Any help on how I could fix that would be appreciated.
Thanks a lot,