PLIIx an eXtension (not improvement) on PLII. They both use the same processing algorithms, so there's no difference in the quality of the surround processing itself.

The primary difference is in the functionality. PLII only works on 2-channel material and outputs up to 5 channels. PLIIx works on 2-channel and 5.1-channel material, converting both to 6 or 7 outputs (depending on number of speakers used).

I think there are a couple of reasons that many people seem to be liking PLIIx. Many are going from from a 5.1-speaker set-up to 7.1 and noticing the improvement. This is also the first time many have heard good quality 7-channel processing. This could lead some to believe that PLIIx is "that much better" than PLII, when really other factors are contributing to the improvements they're hearing.