Depends on the SSD you currently have and the M.2 drive you intend on upgrading too. I swapped from a old 1st gen Corsair SSD to my current Samsung 960 Pro a while back and i noticed a decent difference. In-fact it was almost as noticeable as when i swapped to SSD from HDD(not quite but not far off).
It definetly worked for me. Got a Kingston a2000 for basically nothing, and (for someone for whom it was my first ever SSD experience, it was a serious change of pace. Sure for those who had something like this before, that may be too budget, but hey when you are on a sead minimal budget, you have to make do with what ya got