I've found in my situation watching on a 42' plasma, that HDDVD is far superior to standard DVD's as well as to broadcasts shown in HD over DirecTv.
I've read before that some people who are using projectors have not felt the benefits of HD vs SD were significantly different.
I've compared King Kong, Polar Express and Corpse Bride from HD DVD to DVD and in each case there is a world of difference, of course there are different qualities of HDDVD as well as standard DVD's, but for the ones I own I would have to rate them visually a 10.

A computer once beat me at chess, but it was no match for me at kick boxing.