As Al's review of the 850 EVO exists in a cat like superposition of being biased both for and against Samsung, perhaps you would like a second opinion.  That is where The Tech Report's review comes in handy, which was published just a few short hours ago.  Their findings were perfectly in line with the others, exactly the same performance as the 2.5" drives but in a nice bite sized form factor.  The only drawback is the size, the new M.2's are missing the 1TB model at the moment.

"Samsung's 850 EVO SSD debuted in December inside the usual 2.5" case. Now, the drive is spreading to smaller mSATA and M.2 form factors. We've examined the new drives to see how the mini lineup compares to its full-sized forbear."

Here are some more Storage reviews from around the web:

Storage