I compared the 20" and 24" iMacs side by side and the difference was obvious. The colours on the 20" change when viewed even slightly off centre, for example grey changes to yellow. At higher viewing angles colours invert completely. Typical of a cheap TN display panel, making it next to useless for accurate photo editing. The 24" display has none of these issues and neither did the old white iMac 20" display, but they both use a much higher quality display.
Bottom line is, if you don't know any better you might like the new 20" iMac display, but if upgrading from an earlier white 20" iMac you will feel cheated. It's a real kick in the teeth for anyone making such an "upgrade".