Just a reminder...
nVDVD != nVidia DVD Decoder.
nVDVD was a standalone DVD playing program from a few years back. It got up to version 2.55, and worked pretty well. Not necessarily better or worse than anything else, but it did come with most nVidia graphics cards of the GeForce4 through GeForce FX generations.
nVidia DVD Decoder is just a CODEC - you can't watch DVDs with it, and nVDVD doesn't work with it anyway (if you have both installed, and are watching the DVD on the nVDVD program, you are using the older CODECs that are built into that instead of the newer DVD Decoder).
I honestly don't know what all programs DO work with the new DVD Decoder (that is, btw, the thing you are talking about having to buy - or downloading a 30 day trial of), but I do know that:
[*]nVDVD does *NOT*, it still uses its built-in CODEC
[*]Windows Media Player 10 *does* - and it looks GOOD! (WMP9...I don't think does)
[*]ZoomPlayer Pro does, as well - that's what Anandtech used for testing with it when they gave the decoder CODEC
such a glowing review. (Note that the review is given in context of the hardware-acceleration the GeForce6 cards can do with this decoder...that is completely irrelevant to anything but performance, though - the decoder will work on ANY DX9 card, including ATI cards)