I've seen several comments from people who viewed Retina iMacs and non-Retina iMacs side-by-side and did not notice significant differences. There are two things to bear in mind in such tests.
First, if you are looking at images, you need to use a "Retina Aware" application, like recent versions of Photoshop or GraphicConverter. If using a web browser, there may be no advantage to the Retina display's rendering of images. A web page must contain special code to render images in a higher than normal resolution. The page must have code to programmatically detect the resolution capability of the display, or there must be a UI control that lets the user set the high resolution mode. Very few web sites currently do either. In the absence of such special code, the browser will assume that you are using a non-Retina display, and draw the image accordingly. This means, that on a Retina display, it will be pixel doubled. That's a good thing, actually. If it did not happen, the browser might draw images with height and width at half the appropriate sizes, and there would be a lot of empty space.
I created a page to demo the Retina display for a friend:
http://julian.marmotandweasel.com/photos/retina_demo.html
There are two versions of the same image. The upper one is optimized for Retina, and the bottom one for a non-Retina display. On a non-Retina screen the upper one will actually look slightly worse than the bottom one. On a Retina screen, the upper one should appear sharper.
You are welcome to look at the simple HTML for this page to see how it works. Feel free to copy the code and test it on your own images. But I ask that you not steal my shots.
The second thing to bear in mind is that the difference between Retina and non-Retina screens may be subtle. If you are more than a couple of feet away, your eyes may not be able to resolve pixels on either screen type, and you may not see a difference.
For more samples of Retina images on the web, visit Lloyd Chambers' blog:
http://diglloyd.com/index.html
I think his site tries to automatically detect Retina displays. Look at the indicator at the upper right corner of the entry page, next to the login button. He has a couple of toggled demos (Retina vs non-Retina) on this page:
http://diglloyd.com/retinapref.html