Collecting cast iron cookware would seem to be an odd hobby, and you'd probably be correct in thinking so, but take a look around at the other things people collect (Pez dispensers, match books, cigar bands, bottle caps, post cards, etc) , and it starts to seem more normal. It's something you can buy relatively cheap, clean up and use daily. A collection you can actually use without fear of hurting it. It's actually a collection that gets better as you use it.
You can see the cast iron items in my collection so far HERE.