I've got a 2004 Specialized Sequoia with Zertz in the seat stays, seatpost, handlebar, and fork.
I don't think it does as much to smooth the ride as larger tires run with less psi.
A sloping top tube bike will have a nicer ride than level top tube bike. It's because of the longer seatpost that flexes more.
Sheldon Brown mentions this in his article about frame materials in the Feb. 2007 magazine of Adventure Cycling.
I've had no problems with the Zertz falling out.