I interviewed kathymack for The Daily Curl, so take a gander at what she has to say about soap, soap-making, and using the bars to clean your head.
On the flip side, I want to quote what Tiffany the hair stylist has to say on her blog about soap bars:
...Which brings to a small derail. I know people are tired of me harping on the excessive use of shampoo bars. If, however, you have porous hair, you are not doing yourself any favors by using them. These bars are fairly alkaline and raise the cuticle, the exact opposite of what people with overly porous hair are trying to achieve. Your goal is to establish a routine and determine the most effective product use for your hair without swimming upstream in the process.
I think Tiffany's observation/warning is the reason kathymack follows her soaping experiences with a vinegar rinse (it closes the cuticle). So, be advised that soap + acidic something = hair happiness.