I just spent over hour at an Apple store with 5k iMac and I'm truly impressed. From my perspective the new 5k panel had excellent color reproduction, very even white across the panel, could not notice any obvious backlight bleed ( but has at well lit store ) and no yellowing. The panel looked much brighter in general compared to other 27" TBD and 27" iMacs in store. All of that said, in my opinion there really is no comparison to the 27" older panels ( or any panel I've ever seen to be fair ), the new 5k simply is superior in all aspects and I haven't even started to rave about much of difference the pixel density makes. The other 27" panels at the Apple store look dull and pixelated when comparing them side by side. Bottom line is the old panels are fine until you spend some time with the 5k. This panel is addictive, I didn't want to get up. ( Had to, my son had had enough ) Would have walked out of store with it but need to CTO one so will have to wait

. It even makes the retina displays of rMBPs look lame with their limited color gamut. So as the OP said "Retina iMac make sure you view it in person before buying!" and come to your own conclusion.