If I had to guess, I would say that the law will be upheld with 6 or more justices concurring and that the majority opinion written by Chief Justice Roberts will work very hard to narrow the legal scope of the ruling -- this law is okay but no others like it. The extended hearings are to gather enough testimony to do that as well as possible. Just my conjecture.
I am not a constitutional lawyer or any kind of lawyer. I find myself in the position of thinking that there should be universal coverage but not through this law. Even opponents of the law who have prosecuted these cases acknowledge that the government could have used its budgetary powers to obtain universal coverage. My own suggestions were expressed in this post from December 2009 after the Senate passed its bill:
Specifically, I'd like to see everyone enrolled in Medicaid via the tax return as the default, unless they can prove that they had alternative coverage. They could then be charged an income-related premium for Medicaid on the tax return. I think this gets us to universal coverage more directly -- there is no need to separately impose penalties for those who violate the individual mandate and no need to provide a complicated system of incentives for those of modest means to be able to afford coverage through traditional markets.
That arrangement is no less constitutional than giving a tax deduction for a contribution to a 401(k) plan. With a sufficiently steep income-sensitivity for the premium, it wouldn't cost much either. The trouble is that the Congress didn't pass and the President didn't sign this bill. They passed and signed one that relies on the government's regulatory powers to achieve universal coverage. And that seems to me to be the reason why this case has gotten to the Supreme Court.