I disagree Brett. M.2 is much more than "meh" right now. It gives the best performance by far for the dollar, has a lower form factor and directly accesses PCIe lanes instead of being limited to the SATA standard.
You'll see decent performance improvements by having your OS on a M.2 drive over a SSD. There is a price premium, but not a lot more and you get substantive performance for that.
Today, a Samsung 1TB M.2 on Amazon is $449.99 USD
Samsung 1TB SSD is $431.84
The M.2 performs 163% better than the SSD for an extra $18. That's not 'meh,' especially on a gaming rig.
http://ssd.userbenchmark.com/Compar...e-M2-1TB-vs-Samsung-850-Pro-1TB/m200815vs3577