I tried out MPlayer the other day, and was quite impressed. I did not try the DVD playing hooks, but I did use the freely-downloaded DivX 5.01 codecs for Linux. That worked a lot better than the previous solutions using the Win32 codecs.
The only complaints were scaling the output window didn't seem to work too well, and that the output window isn't directly sizable.
In the past, I've used xine for playing DVDs and it was not bad either.