Mylar D's are GREAT.  I've been using them for 5 
years and they have kept my collection up 
nicely.  However, I would strongly suggest 
backing your cels with acid-free paper when 
using these bags.  Over time the paint may 
adhere to the bag itself, which would make a 
horrible mess of your cel.  

I've never had the paper stick to the cel, 
except statically, which is easy to remove.

jINx