You can use it as a bootable device if the motherboard supports it. You don't have to have a PCIe adapter, but you do need an M.2 slot on the motherboard. Using the M.2 slot usually disables another PCIe slot (on my board it's the very bottom slot).
You also need to have a UEFI BIOS and an OS that supports UEFI (Win7/8).
Basically, as far as I know the X99 chipset will allow this and there are some earlier chipsets that work with BIOS updates.
Motherboards are all different so you would need to figure out what supports it.
I have a Samsung XP941 booting Win7 and it has 978MB/s seq. reads. It's pretty dang fast. However, an 850 Pro on SATA 6Gbps isn't exactly slow either.