Well, for optical drives reliability(Write and Read) is more important then speeds. at least for me.
Higher the speeds , more the jitter, and more chances of data getting corrupt.
I mean when you are backing up or copying important files, The reliability of the data is more important than the few extra seconds ...??
I always use 24x or 32x speed to burn CDR's and 8x for DVD's.
I dunno why, but somehow i "feel" the data will be more "reliably" recorded at these reduced speeds.
on the other hand if you are copying / pirating movie files than a few missed / corrupt bits will not make of a difference since the decoder software's error correction algorithm will take care of it.