Greetings,
  I myself own two sets of both the Hand Painted 
and Machine Printed ones.  (Why have one when you 
can get two at twice the price! *g*)  Indeed as 
the previous post said, the machine manufactured 
ones they appear slightly thinner and are 
slightly transparent.  I myself prefer a black 
background for it. =)
  As to the handpainted ones (there are actually 
6 cels with rumors of two more different cels to 
be released by Gainax), they are in my opinion, 
just as nice as some production cels I have 
seen.  Being specifically made for promotion, 
these seem to have less wear than some cels you 
might buy.  Sort of pricey depending where you 
look but well worth it if you're a fan.
  As to actual production cels.  They're out 
there.  Whoa boy are they out there.  Way out 
there price wise.  Expect a price of at least 
half a Grand for a good one.  You can probably 
find one pop up every other month or so.  Good 
ones are really rare.  In all my searching I've 
found maybe three good cels (1 of Misato and 2 of 
Asuka).  Last one I saw was a set of 4 or 5 
authentic production cels with signatures of the 
creators on a seperate sheet go for about 5 
grand.  Typical economics eh?  Low supply, high 
demand = Cel collector's tears.  
  Anyways, hopes this helps.