I'll start.

Original chain was very stretched out and worn out. Mix and match of several different (9 speed) chain types. Didn't feel too great. Friend replaced chain with an 8 speed. Was better, but never felt right. Ended up putting on a KMC 9 speed chain the other day; what a difference. the drivetrain feels brand new. Smooth, quiet, significantly better power transfer, much better shifting. It even goes around corners faster and smoother while pedaling. Very profound improvement.

Go!