I've now replaced all four of mine, and it made a world of difference in the "smoothness" of the car. I used to get a lot of engine vibration transmitted to the steering wheel, dash, floor board, etc. New mounts all around removed nearly all of the vibration. I think the motor mount you are referring to is the most expensive of the four, costing a little over $100 for the part alone. It took me about 30-45 minutes per mount to replace them, and I'm not a mechanic.
