I have them and I currently have way too much slop in my rear mount. The vibration is about the same but I just didn't get what I wanted from it. Ill be filling the mount or buying a set of billet ones.

*Edit* I have to say though, that I have this problem because I am using a hydro to cable conversion and only have 3 mounts, and only one of those has the insert...and that one was a little cracked before I put it in, i can only imagine its worse now.

With 2 inserts in and all 4 mounts I did notice a difference and I liked it for the relatively cheap cost of the inserts. But if I weren't worried about vibration id spend less and fill them in.