I have a set of carbon bars and a carbon seat post, and I ride an aluminum hardtail, that is extremely beefy.
Everyone at the bike shop swore that I would notice a huge difference, I did notice a difference, but it was fairly slight. I run my Reba Race fork pretty plush, and I have Oury grips (very thick for those not familiar). That might be why I didn't feel it right away. I really notice it after a long ride at Flatwoods. They were much lighter.
The seat post is a different story, I noticed it right away. I have a compliant seat with titanium rails, and the two together make a big difference. I have always tried to use my legs to absorb the shock. These parts make that much easier and less needed.
Also both the bar and post are Bontrager (Trek) so they have a lifetime guarantee, even if you fall.
That is my opinion, but I am a hardtail freak.