A new Nikon D5 review with sample photos, ISO comparison and more was recently published at DCView/Google translation. DCFever also has their Nikon D5 review out (Google translation):
Another D5 ISO test can be found here (link to the beginning of the article):
Update – two more ISO comparisons, first Nikon D5 vs. D4s:
Next, Nikon D5 vs. D750:
The Nikon D5 is scheduled to start shipping at the end of March and can be rep-ordered here.