Nate Silver piles on (reasonably):
Ryan's budget consists of a series of proposals that would cut spending on entitlement programs in a very serious way, coupled with an equally serious restructuring of the tax code that would have the effect of lowering taxes on most individuals and businesses. On the spending side, one can debate whether or not the cuts that Ryan proposes would be (a) wise and (b) politically tenable, but they would certainly reduce the debt in a very substantial way; nobody really doubts that.
The problem is on the government revenues side, which Ryan's tax cuts would reduce, thereby counteracting the effect of his ambitious spending cuts. I'm not going to spend 200 words qualifying this: yes, tax increases have deleterious effects on productivity, which means that you're giving back some number of pennies on the dollar -- and tax cuts have beneficial ones, thereby making the opposite true. But, given that current levels of taxation are low-ish by modern standards, and that Ryan's budget would make them lower, we are nowhere near the point on the Laffer Curve where tax cuts would have a net positive effect on government revenues. (In the long run, at least: tax cuts are more beneficial during a recession when a multiplier may be in play.)
Where the deceptiveness comes in is in the CBO's scoring of Ryan's proposal, which makes it appear as though Ryan's plan would substantially reduce the debt over current projections. The problem, as a separate Krugman blog post points out, is that the CBO actually only scored half the bill: they gave Ryan credit for the spending cuts, but did not deduct any points for the revenue reductions resulting from his changes to the tax code.
And Ryan provided no objective alternative to judge the revenue numbers, even though he insists that he'd raise taxes if his magical realist supply-side theory didn't work. My view, I guess, is that Ryan's spending cuts look promising. His tax cuts look like a replay of Reaganomics, whose failure could not be clearer in the soaring budget deficits today.