If it's your car, you WILL feel it. I only say this because i'm assuming those that are doing this spend a lot of time in/with there car and notice every change that happens to it. For any one else that sits in your car, it's not really noticable except at startups it's a bit more noticalbe. I really notice it when i'm driving alone, and come to a stop, in gear (ATX here), and can see the passenger seat shaking quite a bit....with weight in it, it's cool, but without it moves a good bit.
I took my girlfriend somewhere a few days after with my friends and we sat in the car to warm it up a sec. My friend and i were talking about the mod and she asked if i had done it yet....didn't even notice a thing had changed...like most people.
Hoping to have mounts on by end of the month, so will have to update how bad it does/doesn't get once that happens.