The budget can be read here. I'll be posting my own thoughts shortly. Ezra Klein questions Ryan's math:

He’s saying that in 2050, spending on defense, on food stamps, on infrastructure, on education, on research and development, on the federal workforce, and everything other non-entitlement program combined will be less than four percentage points of GDP. Consider that defense spending has never fallen below three percentage points of GDP, and Mitt Romney has promised to keep it above four percentage points of GDP. Ryan has not outlined a realistic goal.

Drum concurs:

This is not a serious plan. I don't care how serious Paul Ryan sounds, or how many numbers he spouts, or how many charts he buries us under. It's not serious.

Peter Suderman focuses on the Medicare section of the plan. Peter worries that Ryan's proposal is too gradual:

In 18 years, more than 60 percent of the Medicare population would still be enrolled in the current system. Nearly four decades from now, large remnants of today's system would still exist, with nearly 10 percent of Medicare enrollees still enrolled. And even that doesn't capture the glacial slowness of the change. Under Ryan's updated plan, many seniors would still be enrolled in a premium-supported version of government-run Medicare. It is hard to describe this as a radical change to the system. And yet as gradual as the transformation would be, the budgetary effects would be significant—4.75 percent of the economy in 2050 compared with 7.25 percent under the CBO's more realistic alternative budgetary scenario.

Which would seem to be a good blend of policy and politics, no? Jonathan Bernstein argues that the CBO score Suderman cites is a mirage:

[T]he truth is that we have absolutely no idea what the effects would be of adoption and implementation of Ryan's budget. Maybe it would really slash the deficit; maybe it would increase it. There's no way anyone could, including Paul Ryan, guess the answer from the information we've been given.

Reihan is more upbeat:

My view is that a 19% federal revenue cap will be very hard to achieve, and that it implies increases at the state and local level. The virtue of shifting the locus of spending to the state and local level, however, is that this de-federalization of various programs has the potential to effectively de-cartelize government. Residents “vote with their feet” in search of more cost-effective government even now, but shifting more responsibilities to the states will give the states more opportunities to pursue different strategy and to serve a wider array of preferences. 

Gleckman wants more specifics:

Ryan proposes big, specific spending reductions such as cutting Medicaid in half and slashing other federal spending (except for Social Security, Medicare, and Medicaid) by nearly 75 percent from current levels by 2050. But his budget still can’t add up without eliminating or sharply scaling back those popular tax preferences. Which ones, it seems, remain a state secret.

Brad Plumer looks at what Ryan would cut:

Over the next decade, Ryan would spend 30 percent less than the White House on “income security” programs for the poor — that’s everything from food stamps to housing assistance to the earned-income tax credit. (Ryan’s budget would spend $4.8 trillion over this timeframe; the White House’s would spend $6.8 trillion.) Compared with Obama, Ryan would spend 38 percent less on transportation and 24 percent less on veterans. He’d spend 20 percent less on “General science, space, and basic technology.” And, compared with the White House, he’d cut “Education, training, employment, and social services” by a full 44 percent.

Edwin Park likewise points out that programs for the poor would be slashed:

The Urban Institute estimated that Chairman Ryan’s block grant proposal of last year would lead states to drop between 14 million and 27 million people from Medicaid by 2021 (outside of the effects of repealing health reform’s Medicaid expansion) and cut reimbursements to health care providers by 31 percent. There’s no reason to think that this year’s proposal would result in cuts that are any less draconian.

Frum worries about balancing the budget by defunding the poor:

[O]f course Rep. Ryan is right: The country needs a plan to move toward fiscal balance as the economy recovers. But notice how much harder the job gets when you exempt Medicare entirely—and when you try to cut taxes at the same time as you seek to balance the budget. The likelihood is that even this coming period of economic recovery will weigh heavily on many Americans. We are still probably at least three years away from full employment. Does it make sense to squeeze those programs hardest and first?

Ruy Teixeira suspects that Ryan's plan will prove highly unpopular and furnishes several charts to prove his point:

The new Ryan budget: dead on arrival in the court of public opinion.

Michael Barone's counterpoint:

The conventional political wisdom is that Ryan's budget is politically suicidal. But conventional wisdom also held that voters would like the stimulus package and come to like Obamacare. Neither has happened.

Cohn's bottom line:

The reality of our fiscal situation hasn't changed: Restoring fiscal balance will require a mix of spending cuts and new revenue. This proposal, like Ryan's last proposal, tries to achieve balance entirely with the former. It's not going to work, nor should it.

Douthat has mixed feelings:

It’s clear that part of the purpose of these budgets — and their greatest virtue, to my mind — is to get Republican lawmakers used to taking tough votes on Medicare reform, so that they’re prepared to take those same kind of votes if and when their party holds the Senate and the White House. But the votes on a serious tax reform and a serious health care reform will be tough as well, and on those fronts the Ryan budgets aren’t doing as much as they could to prepare today’s G.O.P. for the work of governing that may await tomorrow.