Yup, I have three cels signed by the original 
manga artist.  The cels themselves are worth a 
*lot* of money, but I wasnt even thinking about 
value when I had them signed. I love the artist, 
and the cels so much that I dont care what other 
collectors think of the value.  I would never 
sell them, even if I had Bill Gates himself offer 
the world, I would not part with these cel :).  
Having them signed just makes them that much more 
special to me.  You should think about what the 
cel means to you before you get it signed.  I 
would only get something signed that meant 
something special to me.  Dont think about the 
monetary value.