Product review (2 stars): Doesn't seem to work
I have a persistent water hammer in my house, probably due to poorly installed plumbing.  To get any pressure from the second floor shower the pressure has to be pretty high (set by the regulator for the house).  The water hammer throughout the house is pretty pronounced.  I bought two to test out.  I put one in the kitchen and one in the guest bathroom.  The one in the kitchen eliminated water hammer from that spigot immediately, but about a week later the water hammer returned.  The water hammer in the upstairs guest bathroom may have gotten less severe but it's still plenty noticeable.

Perhaps my application is simply too severe for these little arresters.  I will have to try something bigger, I guess, or maybe I'll install both my current arresters in series upstairs to see if it gets better.  But this may not mean much if the one downstairs has already failed.