I've watched some TNG at my Mom's, who has a big plasma 1080p. It didn't look nearly as bad as some of the currently broadcast low-def shows like Friends
and the like -
use a bunch of new incidental ship shots, but I would be aghast to see CGI remakes of the custom ones they did, like in Where No One Has Gone Before.
Those are ingenius pinnacles of that art form, whereas with TOS, that form was much less mature (and IMO much easier to stomach replacing).
One worry I have with high def TNG - Data and Worf. Especially Data. In the films, closeups of Spiner looked pretty bad. It's definetly a can of worms. Like others are saying, it may come down to some kind of yet uninvented upconversion, and (hopefully) a larger palette of ship exteriors.
That being said, it doesn't look bad
on a highdef TV. Other shows look a lot worse to me. Maybe because the lighting of TNG is often so dramatic.