A good set of lightweight carbon wheels are indeed noticeable, especially on climbs and instant acceleration. I didn't really believe it myself until I put on a pair of tubular Zipp 202s. Even an extended climb of 12% grades aren't as brutal as before, and they go from being something you dread to something you look forward to. That was the biggest difference for me.
Previously I had a set of Mavic Ksyrium SLRs. They were light for clinchers but whenever I ride these wheels they feel like a bag of bricks.