This. Adoption numbers has got to be dead low in comparison to the size of the SATA market. I'm not planning on upgrading my platform any time soon but, I do want a 512GB SSD. For reasons like that, M.2 in general isn't taking off. Plus, the only time I could see myself wanting a small SSD like that would be for a laptop or a low profile mini-ITX build. Beyond that, other than benchmarks, there isn't a whole lot of justification for it in my opinion.
My justification was; Newegg had the Intel 750 400GB on sale for $300 (.75/GB), I paid $.70/GB a year ago for my 850 Pro 256GB (now $.47/GB), and storage speed was the only real bottleneck my system had. I too had deep reservations about whether it would be a complete waste of money, but besides an expensive upgrade to Haswell-E ($1000+), which I can't really afford and don't even like, this was my only upgrade option for this system.
So I took the plunge, and was pleasantly surprised to discover that the 750 is fast enough to actually feel the difference (after using the fastest SATA drive for the past year). The difference is subtle but definitely there. Programs, games (and levels), Windows utilities, Windows Update,etc all loaded a little faster. A few things run a lot faster, like WinRAR, virus scanners, Windows drive cleanup, defragmenters, photo and video editors. I'm definitely glad i bought it.
Anyone who has a Z97, X99, or Z170 board can boot an NVMe drive on Windows 7 SP1 or later, using a PCIe 3.0 x 4, 8, or 16 slot and the latest BIOS (an adapter for M.2-to-PCIe 3.0 x 4 is <$30). Running on my Z97 system with a 780 Ti in the top slot and the Intel 750 in the 2nd slot, the board has to split the lanes into x8 x8, so the graphics card now runs on 8 lanes (but suffers no measurable performance loss thereby, no card can saturate even 8 lanes of PCIe 3.0).
The number of systems that could benefit from this upgrade is increasing every day, and we should see a lot more options in 2016, and lower prices.