Pros:

• Save HUGE amounts of time prototyping large-scale sites with auto-populating navigation, templates, styles, and reusable clips
• Fast and efficient, feels like desktop software
• Many workflow options; review with built-in tools or export as HTML

Cons: None, really. I think this is the best solution of it's kind right now.

Summary: I work as part of a small interactive design agency and we have used Protoshare on a few dozen projects over the last two years. It's cut our wireframing time dramatically, enabled our clients to see and interact with more pages, and helped specify tighter requirements. Pages are interactive and can be as high-or low-fidelity as you need (stock components, CSS styling, or even import images/comps). There's been no downside for us--highly recommended.