What is your opinion on the retina screen for reviewing pictures? I want take photography to the next level and keep my kit light. A small HD-screen with accurate rendition for reviewing is then very handy.

I tend to simply go places and make pictures on the go. Sometimes I go for themes. I am always walking around though.

Resolution (as in pixel density and all that) is not the only consideration. Color accuracy is huge. Depends if you're just reviewing to see if your picture turned out ok (in focus, didn't cut things out of the frame) or if you want to edit or review edited photos.
To see how it holds up to the competition, check it: http://www.anandtech.com/show/8701/the-google-nexus-9-review/8