I have terrible eyesight (somewhere between legally blind and failing a drivers' license vision test) and I find my eyes are usually less tired at the end of a day working on my retina MacBook than they were after a day of using my old 1440x900 13" laptop.
Smaller text is more legible, and larger text is less jagged around the edges so both seem to be less of a strain on my eyes. My thinking is that the sharper the image displayed, the less work my eyes and brain have to do to fill in the gaps.
This could all just be me subconsciously trying to justify the purchase (although I have no regrets about it) and obviously YMMV so I'd suggest going into a store and doing a side-by-side comparison using an application that has been updated for the retina resolutions.