It works, depending on how badly stuck the sketch 
is.  The longer you leave the cel in there and 
the colder the freezer the quicker it goes.  I've 
had success with slightly to moderately stuck 
cels.  I didn't bother with a cel that was 
completely stuck (the cel itself was totally 
painted on all of the acetate).  I'm not so sure 
it'll work with something like that.  But don't 
get the wrong idea.  I didn't go to my freezer 
after an hour and the two were magically 
separated.  I still had to peel one from the 
other ... the freezer just made less resistence.  
You still might lift some paint if you're not 
careful when you peel the two.  

Just my experience ... hope it helps.