The redone scenes in Kai are a very very mixed bag. Most of them do look bad, but you can find a few here and there that look pretty good:
Note that in these, where it actually looks nice, they bothered to faithfully represent the line quality of the original drawings (or made at least a mild attempt to in the case of Gohan and Goku in the ROSAT). Most of the time, the traced scenes have completely uniform line-width which doesn't look good with Toriyama's style (especially when immediately preceded and succeeded with shots where the line quality is intact). Additionally, the art in all of these shots was pretty good to begin with.
The redrawing, when handled well, can really make the old art, where it was good to begin with, shine. When not handled well, it can make the old good art look worse:
And the absolute bottom of the barrel is when the redrawing wasn't handled well
and the old art was bad to begin with:
And then there's the "OK" shots which really aren't one way or the other:
No matter what, splicing cel and digital animation is going to look weird. They could have added synthetic "film grain" with a simple noise filter on the digital stuff and it probably would have looked slightly better (i.e. less jarring)...but they didn't. Oh well.
EDIT: Wow, a bunch of posts popped up while I was making this one. To clarify I was commenting on GravediggerNALK's point #2 (and ABED by extension I suppose).