The Plextor M6e M.2 SSD Series comes in 128, 256 and 512GB models and for those lacking a M.2 slot you can opt for the model below which ships with a PCIe 2.0 adapter for an additional $60.  One caveat that Legit Reviews offers immediately is that for many models of motherboards you must manually enable the M.2 slot in the UEFI, otherwise your drive may not be detected.  Once enabled properly and benchmarked the performance was found to be in line with the advertised speeds of 770MB/s sequential read and 580MB/s sequential write speeds for the 256GB version.  It would seem that the SATA 6Gbs limitation can indeed be overcome but of course that was not enough for the crew at Legit Reviews, they picked up a second M6e and RAIDed them to reach 1408MB/s read and 1098MB/s write!

"Are you wanting to get beyond 550MB/s without having to do a RAID setup? Are you willing to try a new interface? Meet the Plextor M6e Series of PCI Express SSDs! Plextor is leading the charge for native PCIe SSDs and has come up with the first readily-available M.2 PCIe SSD on the consumer market. Other drives like the Samsung XP941 series have been around much longer, but they are OEM only and aren’t really meant for end users. Plextor has stepped up to the plate with a drive that had end user firmware updates, an impressive 5-year warranty and mouth watering speeds."

Here are some more Storage reviews from around the web:

Storage