I also don't know how much hype is in the inserts, but here is a review from '04 Cycling News by someone who took his Roubaix to the cobble stone roads of Roubaix France to check it out. I assume he compaired the Specialized Roubaix to other bikes. :
"Admittedly, I was rather skeptical about the Zertz concept and how much of a difference some clear elastomer blobs in the frame could make to really damp shock. I'm pleased to report, they live up to the hype. While nothing will save you from the Arenberg Forest, riding on most sections of pavé was a considerably less jarring experience. We're not talking full suspension here; you'll still feel all the bumps. The difference is that the vibrations become more of a series of steady thuds than harsh knocks to the frame/body."

So at least this "expert" was convinced.