I got my bow in a trade last year and it had monkey tails on it, so I thought I'd give em a shot for the year. I tested them on and off and they did help dampen the "string vibe" sound a tiny bit, and I didn't lose any speed through a chronograph so I just left them on. They lasted 8 months with a LOT of shooting before I took them off. The ones you buy in the pack are much more durable than the ones that come on mathews bows. The durability probably has a lot to do with how much stuff they contact while bouncing around on the string.
What made me stop using them was they started fuzzing up my cables pretty bad. After taking a slo mo video we saw that they stretch out a ton on the shot and hit the cables. Look near the top cam in front of the quiver hood.