Well just to throw in my 2p ~ I noticed a very real increase in speed. Windows startup was greatly reduced, and apps now just appear instantly. But I have super fast ninja reflexes and no one else on this forum does, also I have Superfectch and Prefetch disabled, which makes a big difference ( if you don't have these disabled then you aren't really comparing the speed of your drive since Superfetch and Prefetch are effectively a RAM cache for you drives ).
In older boards they may run well below top speed, unless you put them in a graphics card slot. I have a number of boards I placed them in, including a Z97, and that seemed just as slow as a SATA SSD until I realised that the M.2 socket was an older one. I bought an Akasa M.2 adapter from OCUK and popped the SSD in to graphics card slot three and it was noticeably faster. But a quick check of the motherboard manual and I realised that I had to place it in graphics card slot 2 to get full speed out of it... and it makes a lot of difference.
Provided that your motherboard has PCIe M.2 sockets then your BIOS will support booting from them even if you place them in a graphics slot.
Personally I swear by them. They are great. However, I would only buy one to replace a SATA SSD coming to the end of it's life or for a new PC. It's not worth replacing an existing, fully working SATA SSD unless you are just looking for something to upgrade and have already upgraded everything else!