I'm experimenting with xrandr's --scale option for this problem. For my external monitor with a native resolution of 2560x1600, I can run "xrandr --output DP3 --panning 5120x3200 --scale 2.0x2.0" to have it act as a 5120x3200 pixel display which is then downscaled to fit the screen. This makes the hidpi-rendered stuff appear at normal sizes on that screen.
My big problem is that there is no support for different DPI settings for multiple monitors. This is quite unfortunate and keeps me from using the full resolution on the laptop's display. Right now I run the laptop display at 1680x1050 (still quite a bit higher than the non-retina 13"), which plays nicely with my 27" monitor at 2560x1440.
Besides the display issues, Linux runs great on the MBP retina (late 2013 MacBookPro11,1). Things run more smoothly than my last Macbook (a mid 2010 13" MacBookPro7,1) which had occasional wifi and suspend/resume nvidia issues. The only thing that I can think of that doesn't work is the builtin webcam, which was switched from USB to PCI-E and doesn't have a linux driver yet.