I think it is a good plan, maybe not what I'd call ideal, but I think it's the best on the table.
Pros-
Simple and transparent, everyone pays the same percentage. Removes a big burden from doing business, investing, etc. Overall taxes are lowered, while the same or more is brought in by making everyone pay in. Also, sales tax ensures everyone pays something, illegals and such included.
Cons-
Like all taxes, it has the danger to creep. I think the sales tax portion should also exempt the sorts of goods most state taxes exempt, such as food medicine and basic clothing.
I think the thing that puts Cain at the biggest disadvantage with the plan is that he is over simplifying it. If he got more detailed such as what he would exempt, where those tax zones would apply, what language would be in the bill preventing increases, etc. he could gain a LOT more support for it.