I use a dCs Purcell. I have found it to improve virtually all of the CDs I've played through it in the past year. When I bought it, my dealer had mixed feelings about it, feeling he liked some discs better with it and some better without. Don't know if he still feels this way. I'm not sure why, but since my feelings are that the Purcell, among other things, improves the bass and makes for a richer harmonic presentation, a CD with an abundance of one or both of those two attributes (a rare thing, in the case of the harmonics) might sound too bass heavy or muddy in the midrange through this upsampler. As I said, I haven't really found this in my listening, which is mostly classical, with some rock, folk and jazz thrown in, except perhaps some earlier Telarcs which had a tilt towards the bass. As for the second part of your question, the upsampler won't save a poorly done CD (try any overcooked pop recording or early digital classical recording from DG or Columbia), and I've found the best recordings (Reference, Harmonia Mundi, Delos) to sound even better, so I think that it certainly doesn't even the playing field between good and bad recordings, and may even widen the field a little (i.e., more improvement on good recordings than on bad ones). Just my observations; curious to see how others hear it.
