In response to Dempa's currently highest rated solution, I would like to add that 200% @ 2880x1800 looks perfect on my 15 inch Macbook Pro Retina, as far as desktop icons, sizes and crispness goes. It looks nice. Some apps/programmes though (Audodesk Inventor in my case), are still fuzzy around the icons (not untilising high res). 150% looks better for those aesthetically, but for ease-of-use, they are a bit smaller, but work-space is bigger, it's a compromise, I'll decide later, up to you really. It might be better for my cad software to have slightly smaller icons, more space, and that crisp look. At least this is a more exact adjustment than before, where it was either minuscule and sharp or big and fuzzy. 200% for me seems to be correct size and full resolution. Unfortunately not all apps are adaptable.
Also I highly recommend improving the crispness of text too, which goes well with retina. Text looks crisp again, including in web browsers. Right-click on desktop>display settings>advanced display settings>Clear type text