I was forced to use my 13 rMBP solely the past few weeks as I was on a trip and away from any type of external monitor. My screen of choice is normally the 27" Thunderbolt Display. Now that I'm back home, I'm finding it incredibly difficult to get used to the DPI of the TBD. I'd almost prefer using the 13" Retina screen exclusively. Granted, I don't have the best eyesight as it is, but I still prefer the extremely crisp screen of the Retina to any other non-retina screen. I'm starting to notice the difference so much now that I am considering a 27" Retina iMac.