If you're going to a convention, they are definitely worth considering. Readers collect these...whether or not they've read your book. The last convention I went to had a contest where if readers collected so many (kinda like a scavenger hunt), they were entered for some big prize. It brought readers to my table to talk to me and even led some to buy my book. Many wanted me to sign the trading cards. Plus bloggers ask for swag from authors all the time, so it's an inexpensive way to get your name--and book--before a reader who might not have heard of it before.