They do sound much better most of the time, simply because the other available CDs with the same material do not sound as good as CDs can.
I only know their jazz reissues. DCC remastered a lot of OJC recordings (Riverside, Prestige, Contemporary, ...) which were rather poorly transfered on the original late 80's CDs. The same is true for MFSLs remasterings of Blue Note and Atlantic recordings (Blue Train, Giant steps, Night in Tunesia).
So basically MFSL and DCC did what was technically possible to restore the sound of the recordings, while the original labels did not really care about the sound.
Today, most reissue labels are much more aware of sound quality and are trying to sell remastered versions, so the difference is not as important anymore.
It is certainly not the gold surface of the CDs that made the difference :)