I made the change to a carbon fork from an aluminum fork on a vintage 91 Cannondale R400. The difference was significant. Much smoother, less vibration with less stress on the arms (due to less vibration). But, it's important to note the fork came from a 2.8 Cannondale that had the same rake as the fork I removed so the handling of the bike was not affected by the change. As was stated before, it didn't help my butt with the stiff ride of that frame.
I just added a carbon seat post to my Cannondale SR500. I didn't expect much of an improvement, it was in the parts bin so I made the change. I was wrong, after 50 miles I could feel the difference, the carbon post stays on.