I liked Cain until he put out the 9 - 9 - 9 plan. For me a national sales tax is the best way for the government to raise revenue. However, before I'd support it, it would have to be coupled with a repeal of the 16th Amendment. In fact, it really should be coupled with an express constitutional ban on an income tax. (The Supreme Court decisions that prohibited a national income tax as we know it today were rendered before legal scholars viewed the Constitution as a "living document." Modern scholars would probably "interpret away" the former constitutional protections against the current income tax system.)
If Cain gets elected and enacts 9 - 9 - 9, then in 20 years, we'll have a high national income tax and a high national sales tax. Worst of both worlds.