I didn't care much for the movie either. The
manga however is wonderful, thus getting cels
from X the movie would be worth it for me because
I love the characters. One day I may get a cel
of Kamui, but I HAVE to find one of Subaru. It's
been my mission ever since I started really
collecting cels. I missed a cel of him once
(despite the several emails I received from nice
people trying to get me to head to Taro's
site...damn the need for sleep). Hopefully
another will come around.
But to your question, the cels aren't that rare
as compared to other series/movies, but they are
expensive (probably do to Clamp popularity and
detailed artwork). However, if you're looking
for specific characters, it could get annoying to
find the ones that don't show up much. Kotori
shouldn't be too bad. There are serveral cels of
her already up on a few sites. In fact Yann has
one for sale that I really like (but it's $570!).
Have fun finding what you want!