Purchased an advance copy of Boortz and Linder's FairTax book. One of the most cogent rebuttals I've heard concerning the FairTax is that in order not to send deficits sky-high, the national sales tax proposed would have to be something on the order of 30% or so… which would inevitably prompt avoidance behavior, further reducing state revenue. Therefore, it will never pass.
I'm not sure of the extent to which the numbers eventually fall into place: nobody is, because it depends on the specific proposal put into law. FairTax is a real piece of legislation, not an incremental wonkish tinkering. Therefore, it faces higher hurdles than the sort of everyday, out-of-sight Congressional business we're accustomed to getting from DC.
However, even if this is true, thhere are four mitigating reasons to support the FairTax proposal.