AFAIK no linux environments handle HiDPI well at the moment. Gnome 3 added support in a recent support but it's still not great. Also many apps that don't use native APIs look bad, Chrome for example is almost unusable.
What are the native issues with retina (in Gnome 3, for example)? Are there GUI elements that don't scale along?
I might try and install Mate and see if it is any better. I have no interest in using Unity or XFCE so I can't comment on those.
http://i.imgur.com/2GpqyND.png
The title-bar size is configured in pixels, so that could be fixed easily by the user, but icons, both in the GIMP and in the native window don't resize well. It seems the DPI scaling is only applied to text, and only text-containing elements are scaled accordingly.
And apparently there is an update today! Can't wait to try this out.