Here James talks about the old AD&D Monster Cards. I currently own 1 1/2 sets of the cards. One was my original and is about 80% complete. Various cards were lost over the years. The other was recently given to me by Tim (of Gothridge Manor) and has all the cards.
I can add little to the details of James' post. The reason they weren't a smash hit with me is that they were not complete enough. The choices were scattered throughout all the MMs of AD&D and the mix never reflected the mix of monsters I used in my campaign.
James commented about AD&D stat block easy to remember. Sure that true, to a point. With 400+ monster floating around there was a lot of note taking or page flipping for me. The monster cards should have focused on the monsters covered in a particular manual. Then it would have seen greater use.
After my experience with D&D 4th edition I would probably use a version of them today as I found cards to be useful for playing sandbox style. Not only they can act as a reference but also as a random table by pulling cards out of a deck.
We did get good use out of it as a sort of a trivial pursuit game. It was considered a fairly impressive feat to remember the name of every monster on the cards.
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