We've seen a bunch of 9-series Intel motherboards with M.2 slots designed to accept next-gen SSDs. In most cases, these mini slots are connected to the chipset, limiting them to two lanes of Gen2 PCI Express connectivity. That dual-lane link still offers 1GB/s of bandwidth in each direction, so it's quite a bit faster than 6Gbps SATA. However, it's not wide enough to exploit four-lane M.2 SSDs like Samsung's XP941, which is rated for peak transfer rates up to 1.4GB/s.
Apparently keen to address that shortcoming, ASRock has equipped its Z97 Extreme6 with two M.2 slots. One has dual Gen2 lanes derived from the Z97, while the other is an "Ultra M.2" implementation tied to four Gen3 lanes from the CPU. This direct CPU link can handle speeds up to 4GB/s, but good luck finding a drive that takes advantage of it. The XP941 is pretty scarce online, and I don't see any other four-lane M.2 SSDs listed. M.2 drives with Gen3 PCIe support are nowhere to be found, at least right now.
Despite the lack of suitable drives, I've gotta give ASRock credit for thinking outside the box—and for making the amped-up M.2 slot available on a relatively affordable model. The Extreme6 is selling for only $170 at Newegg. It's loaded with other goodies, too, like a SATA Express port, amplified integrated audio, and upgraded electrical components. The board even has a Mini PCIe slot primed for wireless cards.
Thanks to its Ultra M.2 slot, the Z97 Extreme6 may be the most forward-looking Intel motherboard around. It also hints at the potential of CPU-connected storage, which seems poised to become more prevalent in future generations.