I like genga, but not as much as cels.  Cel is
colorful, and it really makes a difference 
when looking at it.  Plus I personally feel when
buying cels, I hope the studio would pack the cels
with original dougas and gengas together.  But of
course, this doesn't happen often.  I only have a
few cels that come with a package deal like this.
Also, I will prefer to buy some well known
animators' gengas, such as H. Mikimoto, N. Yuuki
or H. Miyazaki...etc.  It will be even better if
it has a signature on it.