Over at NRO, Duncan Currie gives it his best shot:
Despite being more efficient than an ordinary sales tax, a VAT carries significant administrative costs, and piling it on top of the present U.S. tax structure would be a mistake. But using the VAT to eliminate a sizable amount of distortionary U.S. income taxes would yield a far more growth-friendly system than the one we have today. Over the long run, America must reorient its economy away from consumption and toward investment while boosting its dangerously low savings rate. A VAT is certainly not the only way to promote those objectives, but it should at least be part of the conversation.