Pros:

• clever, practical and useful
• makes it so easy to step through user-flows
• high-level wireframe elements are the best, but the hi-fi elements are necessary for people who can't visualize

Cons:

• Clients/stakeholders are going to ask to - make a lot of changes and you'll be tempted to write on the cards to keep track of all of them??

• people who can't visualize or who don't understand concept of wires will get hung up on the design of the hi-fi elements, but that's a universal problem for UX