Plextor demonstrated something this year in Taiwan and other places, but the final product is coming to retail only now.
Samsung's PCIe cards are inside Sony's ultrabooks and Apple's desktops and laptops, but not sold to retail on any place I know.
The Plextor guy visited Seoul recently to announce M6e price woud be more reasonable, which I understand to mean anywhere between US$150 and US$500 for 256GB version. There are only 128GB and 256GB versions for now. The performance is similar to that from Samsung's own XP941, but Plextor uses Toshiba's 19nm toggle MLC NAND chips and Marvell's 88SS9183 controller.
What I like most about these new PCIe cards is that I see them becoming like ISA and PCI Ethernet LAN adapters, pricewise, that first appeared as expensive gears for some new technology called networking and even connecting to new space called TELNET and FTP. ISDN adapters that could be installed inside of PC appeared next and I bought some of them, too, at half the prices of standalone ISDN modems. Introductory prices are prohibitive, but that's because they are rare and manufacturers want consumers to pay for premium trying such an unprecedented, untested thing. PCIe prices will become normalized to about half of 2.5-inch form factor counterparts.
Pictures borrowed from South Korean sources: