I have already had 2 cels get damaged (one was
more than $100).  I live in a hot, humid climate.
 So the 2 cels got stuck to the cel back and when
I tried to remove it, the black lines (on top of
the cel) peeled off!!  I was practically in tears!  
Since I have recently purchased some sketches, I
use them for those since they have no paint and
they are much more durable than the other cel bags
(and they tend to enhance the sketch lines).

I have been using those Heiko (?) bags from Cel
Mart and have yet to have a problem.  As a matter
of fact, I usually use them for cels without their
sktches because I noticed that they have a less
tendancy to stick to those cel bags than the
sketches (I have yet to have a problem with them
getting stuck to the cel).  So those I would
definitely recommend.
Hope this helps a bit.

Star Phoenix