From my own experience, gotta tell you that I haven't seen any improvements whatsoever, so the best feature for me on these upcoverting players is their ability in tweaking the contrast, gamma, hue, etc, without touching those on your display.
Bottom line, IMO, if you have a TV with a good scaler (my Panny does), there's no reason for another scaler in the chain at all.
That might be different with HD players, though
My $ 0.2