Having grown up in Washington State which only has a sales tax, what I consider to be the bad side is that unless you save every receipt and total up what you spent in a year, you don't really know what you paid out in taxes.  I would much rather know so that I can decide if I think it was a reasonable amount for what the government provides.

I do favor a flat tax both on a state and federal level.  Apply it to all income, regardless of where it came from, and eliminate all deductions (including child deductions).  The government should not be using the tax code to encourage or discourage individual behavior and to reward political supporters and punish political opponents, something both Republicans and Democrats in Congress do as a routine matter of business.