I've got a 20" iMac G5 1.8Ghz, and the screen is incredible. Great viewing angle, very bright and extremely crisp. It's been great having such a big screen. The panel in the iMac is the same as the cinema display, so they should look identical. The ADC connector is used only to clean up your desktop, and carry power, video and USB through the one connection. Note that none of the new Power Macs sport this connection anymore, so you'll either have to get an adapter or already have a computer with ADC.
jared