I recently bought a Cervelo R3, but was considering a Specialized Tarmac during my shopping. The Tarmac had those Zertz inserts on both the stays and the seat post. At first I was intrigued, but then brushed them off after considering their potential from an engineering perspective.

I am a structural engineer; as such I occasionally design building frames with dampening systems incorporated, when in a seismic region. The principles of dampening on a bike really aren't much different than dampening against seismic motion in a building. Anyway, as I see it, here is the gist of it:

Dampers work substantially to gradually absorb vibrational energy when the input energy is unsustained. Consider a shock briefly applied to a bike frame; a very resilient and stiff material like aluminum can allow the frame to continue its vibration for an extended period of time after the shock (riding over a bump) ends. When a dampener is introduced in the system, that device will gradually absorb the energy and cause the vibration to more quickly diminish. A good example would be to contrast how long a tuning fork can sustain its free vibration between the case of unbraced and braced ends (ie: touching the vibrating middle with your finger, which acts as the dampener - this is akin to the zerts inserts).

However, dampeners don't do much if the input energy is continuous. Well, such is the case of rough road surfaces and repeating road ripples. Frankly, I am not too concerned as a cyclist with infrequent unsustained shocks, such as from dips in the road - I just lift my seat and ride through or around them.

In short, I think the zertz inserts are a marketing ploy. Specialized has exploited a scientific principle that probably has little impact on the performance of the frame. In fact, their inclusion might do more harm by introducing a weak point in the strength of the frame, etc.