I'm not that big a fan of mylar bags mainly due 
to the strong static cling.  As mentioned prior, 
the static cling is strong enough to remove 
specks of paints from the periphery of cels.  
Paper of any kind, even backing tissue/rice 
paper, will stick to cels over time.  I'm also 
not a fan of the ultra glossy, oily sheen 
presentation of mylar bags.

Yes, mylar basically lasts forever and doesn't 
warp, but I use thick polypropylene sleeves (open 
on both ends, no [sticky] flaps to worry about) 
from iconusa.com:

http://www.iconusa.com/online/SafteySleeving.htm

It's the set of 50 that costs $10.50.  They do 
fit nicely in the 11" x 14" Itoyas as well.  
Because of the thickness, it would take much 
longer for the sleeves to warp.  I've had some 
sleeves for a little under a year now and I 
haven't seen any warping yet.

I've also cut up and used the sleeves as padding 
between cels and bgs and to make bigger 'bags' 
for my big cels.  I used about 3-4 sleeves to 
make a bag for my 14" x 22" miyu cel, heh.