....is SO disappointing! I picked this up from my local game store today only to find out that the Gloom card inside is non-transparent! The card isn't even made with the same dimensions as regular Gloom cards, so it doesn't shuffle easily into my game deck. On the other hand, since it's a character/guest card, I guess it's not entirely useless. It's a nice little token, but that's all. For anyone wondering what Gloom card comes with the Duck of Gloom, look here: 
My initial reaction after opening the cards was a bit of shock and disappointment. Yes, it's not transparent and not the same size as Gloom cards; but as a guest/character card it ultimately does not negatively affect game play.
After playing a few games with it, I think I've actually come to appreciate how this card stands out - a reminder that it's more of a bonus/optional card. That's just my experience though. Other players may not be able to get over the format of the card like I eventually did. But at least now they can look at the card and decide for themselves without being surprised.
I appreciate the effort and I still enjoy both games. I wish more of the makers of other games I play would collaborate more like this.
Hi there. Because Duckworthy is a guest, he doesn't HAVE to be transparent, as you noted ... he'll always be at the bottom of the pile of mods. The size had to match Munchkin, unfortunately, but again it doesn't actually impact play. I'm glad you decided you liked it in the end, since Duckworthy's reception will impact whether we decide to do more in future, and using opaque cards makes it a lot more affordable to do. 
If you haven't seen it already, check out the preview for the card, with commentary from designer Keith Baker: atlas-games.com/gloom/DuckPromo.php
Sir Duckworthy interested me enough that I picked up Gloom. I enjoyed watching Wil and company play and look forward to actually playing myself.
Thanks for letting us know!
I still have mixed feelings on it. On one hand it's a pretty cool card for the game, on the other hand its non conformity makes my inner gamer scream. I guess it still works since you don't shuffle it in, but the not matching was still disappointing. On another note I'm really surprised there aren't more promo cards for Gloom.
That's because printing on plastic is so expensive. We can't do it unless we gang it with a huge reprint of Gloom and/or its expansions, like we did with TableTop Gloom, and even then it's not really financially viable in most cases (TableTop was a special case, obviously).