the difference felt between "ride quality" is is highly dependent upon many variables as others had said. wheel/tire choice, psi, yada yada.

I think it comes down to differences in 2 particular bikes, so results will vary. to say carbon is smoother than alum is an invalid statement without identifying which carbon and which alum.

In my case I went from a my 2005 lemond 6066 alum to my 2010 carbon kestrel. I transferred all my components from the lemond to the kestrel so I think I have a valid comparison which is only looking at the frame and seatpost. heres my impression.

while the lemond rode nice, comfortable, I was able to feel a "bounce" in the frame when on rough roads. while climbing, really putting power down, I could again feel that bounce. it actually became part of my pedal stroke, maybe it helped me set cadence and make it up the hill. The bounce could be best described as hitting a bump, feeling a rebound, followed by 3 or 4 more rebounds that diminished in strength. like dropping a basketball on concrete.

the kestrel also rides very well. rough roads seem a little more harsh. I do not have the diminishing bounce at all. the initial hit is felt, maybe slightly stronger, and then its completely gone, no echo. I will definitely say that when Im climbing, putting down the same power, this bike jumps to life. My legs feel more connected to the drivetrain. No bounce at all. suprisingly though I dont feel like my body is getting beaten up or jarred by the end of my ride. cant explain because if its a stiffer frame then the shock should be absorbed by my joints and muscles.

I prefer my carbon frame over my old alum for many reasons. the transfer of power well outweighs the slight comfort advantage of my lemond.

the saying goes, "high end alum is better than low end carbon" I believe that %100. I will say high end compared to high end, carbon rules for efficiency.