There seem to be a lot of configurations with problems, although the nvidia drivers work great for many, too. As usual, you get to keep all the pieces if something breaks.
Especially support for older cards is fading - there are 4 release series from nvidia for different sets of cards, which already tells the story that they are not able to support everything properly. Of course the older ones of those series seem to have less support when new kernel / X releases are done, it's a bit of a guesswork.
I have access to one machine with Geforce FX 5700 card, and none of the drivers work really properly for basic compiz + games + video stuff. I hope Nouveau [http://nouveau.freedesktop.org/] will save NVIDIA users before the support fades completely away for older cards.