I recently went from an 850 EVO on a SATA 2 port to an HP EX920 m.2 SSD, I can hardly tell any difference at all. In benchmarks there is a difference but in real life, I would not consider it a worthwhile upgrade. I did mine mainly because I wanted to use the 500GB SATA SSD in a laptop and it just made sense to buy another 500GB SSD in M.2 NVMe instead of SATA. In hindsight, I kinda wish I would have bought a larger m.2 drive like 1tb but that simply wasn't in my budget at the time. Just one thing to note, I think most PCs can not use a PCIE > M.2 adapter as a boot drive. I might be wrong, but everything I have read says this doesn't work.
My Asus Z97-M Plus has been running a Samsung SM951 NVMe drive since new, when I first installed it I needed to load a driver at install but iirc since then Windows 10 has been updated to include an NVMe driver. I used it to reduce clutter in the case rather than for speed purposes.
