Balsamiq fits well to most iterative needs atleast in my experience.
I am talking exercises where you are tweaking, changing an already sketched experience or work flow.
When it comes to wireframing for an experience from scratch, my first choice remains pen & paper, simply because I know that tool for 20+ years now and it gets my creative juices flowing :)
After the basic sketches are done and critiqued by every "design conscious" person (read everyone in your organization), its usually time to migrate that work to Balsamiq for future iterations.
