Back in 2017 I bought a carbon Trek Domane with front/rear "ISO Speed" vibration damping. It was noticeably smoother on typical pavement I ride on here in Maryland, which is rarely fresh, smooth ashpalt but not a lunar surface either. Hit a pothole, it still feels like a pothole but the little bumps/buzzes are definitely smoothed out.
My previous bike was a steel Trek 520, that was the baseline I was comparing to. Both bikes are runing 32mm tires.. The Domane came with padded bar tape - that also makes a difference up front.
Last year I did the Seattle to Portland STP ride, 200 miles over two days. I rented a Fuji aluminum road bike that had geometry pretty close to my Trek and I brought my own seat, so using the same seat. The first ride I felt like I was back on the 520 from a bumpy road point of view. A few hours in and I didn't notice it anymore.
So, noticeable - Yes. Does it really make any measurable difference in speed/efficiency/fatigue? I sorta think it does - but hey, 30% of the time placebos work, too.
All my training for the STP was on the Domane and I was in pretty good condition but still carrying 225 lbs of me around. I did 122 miles on the first day on that Fuji (my one day high) without any of the fancy vibration damping. and felt great. YMMV.