I have never actually bought one, despite wanting one for quite a while. It's a stiff price and regular buffer/spring work fine.
That said, I think it makes the rifle incredibly smoother and love the feel on the few guns I have tried with them. It eliminates the "sproing" sound of the spring, which I actually never notice but it bothers some.
At some point I am going to bite the bullet and buy one for my rifle with a low mass bcg.
It's not a necessity, but its a super nice part. Like all JP parts. I am a big fan and when I hit the lottery I am ordering one of them fancy JP rifles to go with my new Infinity pistol!
Probably stop talking to all you people at the same time. I keed I keed.