Lately (within the last year or so) I find myself 
really preferring gengas to cels themselves.  I 
think it's because there can be a lot of detail 
in a sketch--even in a douga--that's lost in the 
transfer to acetate because of gunky Xeroxes or 
carelessness in the painting.  Having a full 
setup with a background is nice and all, 
especially if it's multilayered, but I think I 
would prefer a nice layout sketch with the 
characters and the background drawn on a single 
sheet. Assuming the art wasn't too sketchy, of 
course.  By the time you get to the cel level, 
the image has been traced so many times that it 
tends to lose the life it had in the earlier 
stages; even though the effect is subtle, it does 
make a difference to me. I'd rather have an 
energetic rough than a bland cel.

Plus, sometimes I think that the colorlessness of 
sketches can work for them.  If a show is based 
off of a manga that I like better than the anime, 
I'll usually prefer to have something that looks 
more like the manga art than the anime art.  A 
lot of times I don't like the color schemes of 
particular characters (for example, I can't stand 
Roronoa Zoro's hair color in the anime, though I 
love him to itty bitty bits and desire sketches 
of him so very badly), or the coloring was very 
flat and lifeless due to budget constraints or 
whatever; in these cases, having something 
colored would be a distraction, even if that is 
how it looked on TV.  It sucks if everything is 
on separate layers, of course, but that's the 
only real problem I have with sketches.

As for whether I like roughs or eye candy stuff, 
it depends on what you're looking at.  I have a 
beautiful set of gengas that include charcoal-
like roughs done by the character designer (I 
think), and clean gengas that have a lot of color 
and fine linework to them; both are beautiful, so 
I can't decide which I prefer.  If a rough is a 
frowny face done by Super Important Animator #3, 
then it won't do anything for me, but if it looks 
pretty, I definitely dig the roughness of it.  
Hard to talk about hypotheticals in something 
like this.