Everything I have read indicates the cranks are essentially the same in terms of stiffness, weight (within about 20 grams, depending on whom you believe), and durability. Carbon cranks cost a little more. They look a lot cooler. I don't think you can go wrong with either.

Unless you're buying / building a new bike, or there's something wrong (read not working) with your Dur-ace cranks, I'd do something else with the $$...Now, please don't read this as I don't like carbon cranks....My 6 month old Felt S22 came standard with FSA Carbon Pro cranks and I have been very happy with them...However, if the bike had come with a Dura-ace or Ultegra crankset, the carbon cranks wouldn't have been on the radar. My $0.02

Have you noticed any creaking, flexing or anything different (mechanically) from aluminum cranks?

No, absolutely nothing different...Except maybe the Q factor is a little different from my old Ultegra 9...Remember, the crank arm is NOT ALL carbon...It is a thin aluminum frame with carbon laid up on the outer skin...If they're stiffer, I can't tell it...No creaking though and don't scratch any easier or harder than all aluminum...I have to admit that they do look cool :)

Carbon looks cool, but performance is not significantly different from aluminum and in some cases worse on a deflection/gram basis.

However I do have the FSA superlight cranks (they have supposedly done away with the aluminum skeleton though I haven't cut them into pieces to verify). They are about 45g lighter than the pro team version and DA, but in actual use the main thing I like with the FSA versus aluminum cranks is there is much more ankle clearance.