I tried their demo but I couldn't figure out how to get the system to work. I also couldn't figure out what language the core is written in. OK, after fiddling with the PHP version of their demo, I figured out how to use it (the Java applet remains a mystery). It works very well. You basically have to select a random image and say it's "relevant", then tune the query from there. Not particularly useful as compared to query by sketch or even Eikon's explictly query image.
The Viper team also has a very nice list of other content-based image query systems.