I considered cards but decided on emails instead. Since a chunk of the communication between me and the client is by email, sending a follow up email works well. I don't need to buy cards, postage, and take extra time to mail. And I prefer a paperless approach. Card or email, I think the important thing is to follow up with the client.
I do think hand-written thank you cards are a nice touch, and I may incorporate it in the future depending on the feedback I get from clients.