Tanquen :
"There's a good chance you won't notice the difference between a fast SATA 6Gb/s SSD and something plugged into M.2. And this is from a guy who tests SSDs all day, every day."
There is a good chance you won’t notice the difference between a “fast” SSD and a hard disk in the day to day life of a PC.
I don't think that's accurate. I have two otherwise-identical laptops, one with an SSD for its boot drive and one with a HDD. The one with the SSD is hands down faster in program launches, file opening, and other read/write tasks.
I've also compared fresh installs of multiple OSs (Windows 7, Linux Mint 16, Ubuntu 14.04) between hard drive and SSD, as well as comparing them to my old Windows 7 install on a relatively slow SSD, and none of the fresh installs on hard drives even came close in speed to the old and bloated install on the SSD.
I'm sorry that you haven't experienced the difference between a SSD and a HDD, I do truly believe it is the single most important upgrade one can make to their PC.