>It's difficult to notice any pixelation on a 27" screen at a resolution of 2560x1440, so clearly the reason to upgrade to 5120x2880 is for the extra screen workspace.
On the new 5K iMac you won't perceive a bigger workspace^ than you had on the previous (2560x1440) model, because all UI elements will be scaled up at the factor of two, practically making everything look twice as sharp.
^: Actually, just like with the retina MacBooks, you'll have the option to use alternative resolutions up to 5K with and without scaling, effectively creating a larger virtual workspace.
On the new 5K iMac you won't perceive a bigger workspace^ than you had on the previous (2560x1440) model, because all UI elements will be scaled up at the factor of two, practically making everything look twice as sharp.
^: Actually, just like with the retina MacBooks, you'll have the option to use alternative resolutions up to 5K with and without scaling, effectively creating a larger virtual workspace.