I can't speak for everyone, but I like 2mil 
Mylar D bags.  They are really stiff and clear.  
The only thing with these bags are that you need 
to buy backing tissue.  If you don't, the 
electromagnetic force can make the cels stick to 
the bag. 

I have gotten Mylar Ds at a reasonable price at 
bagsunlimited.com.  I don't remember where I 
bought the tissue, but I know you can find some 
at archival specialty stores. (Oh yeah, make 
sure you get acid- and ligen-free kind!)

I can honestly say I that compared to the other 
bags I've used (the ones that dealers typically 
use) that my cels hold up a lot better in these 
bags. (i.e. no layer spotting, dust, etc.)

Feel free to drop me a note if you have any 
questions. (jINx@jcrave.com)

jINx