The SSD Review published a review on the ASRock Z97 Extreme6 Tests Samsung XP941 M.2 x4, Plextor M6e M.2 x2 and Samsung 840 Pro SATA 3 SSDs A quote from the article:

To anyone not familiar with SSDs, our title might not seem that striking, but it represents a huge step forward in storage technology within a very short time. It also speaks highly of ASRock for again leading the way in advancing storage, as they are the only to incorporate a new SSD innovation into their Extreme 6 motherboard; this of course being a PCIe Gen 3 x4 interface that could accommodate M.2 SSDs up to blazing 32GB/s speeds. Although it may be some time until we get such a small form factor SSD travelling at that speed, this does mean that the Extreme6 is the only motherboard in the world that can accommodate our Samsung XP941 as a boot drive today. In fact today, we are going to be testing not only that XP941, but also the Plextor M6e PCIe x2 SSD and the Samsung 840 Pro SATA 3 SSD.

 ASRock Z97 Extreme6 Tests Samsung XP941 M.2 x4, Plextor M6e M.2 x2 and Samsung 840 Pro SATA 3 SSDs @ The SSD Review