Do these things actually work or is it a waste of money?

Not sure but I have never shot my M1A or FAL without one. Haven't had problem yet.
These rifles are intended to last forever without a buffer. With a buffer it will last forever easier. It just makes sense to me.

I think they are great in the AK type rifles. The bolt assembly on the AK REALLY slams into the back of the receiver.
I put one on my M1A/M14, it buffs the op rod instead of the bolt.
I like them on the 1911 also, but the 1911 will chew them up after a while. if you don't replace them, they can gum up your pistol.

I put one in my (now highly modified) Maadi before I even fired it for the first time.  I'd rather beat up a couple-dollar piece of polymer than the receiver of the rifle.  Similar items have been used in 1911 type pistols for a good while now.  The consensus seems to be that if it is a gun you are betting your life on, you should maybe skip the buffer.  In the pistols, at least, heavily worn buffers have been known to come apart and cause malfunctions.  In guns that see a lot of range use, they are a good idea as they save wear and tear on the gun.

I put one in my mini-14, before I ever fired it.  I did not like it so I took it out.  I pit one in my AK, and it works great.  I have not seen any deterioration in over 1000 rounds.

Why? did your mini jam?