Legal Opinion: I personally believe the ACA is constitutional and should be upheld. The most controversial component, the individual mandate, is no different than the mandate that we all contribute to social security or from the state laws requiring that we purchase automobile insurance. And so, if the case were to be decided on strictly legal grounds, it would almost certainly be upheld. (18 of 21 legal experts surveyed agree). Unfortunately, several of the Justices are more inclned to rule based on partisan politics, and so the legal arguments are more or less irrelevant
Practical Analysis: Of the nine Justices, the most predictable is Antonin Scalia. He always rules based on his extreme right-wing ideology and never worries about the law, logic or precedent. I predict that he will vote against A.C.A. Justice Samuel Alito (sometimes called Scalito for his almost absolute deference to Scalia, will follow suite. To round out the tea party triumvirate, Justice Clarence Thomas (who hasn't had an original thought since he's been on the bench) will do whatever Justice Scalia tells him to do. Hence, we have three almost sure votes to overturn A.C.A.
On the other side of the issue, we have Justices Ginsberg, Breyer, Sotomayor and Kagan. In my opinion, they will follow the Constitution and vote to uphold the law, even though the tea party will say they are knee-jerk liberals. Since I am a private citizen, I am allowed to have a bias and so I agree with the position of the righteous four.
This leaves two swing votes, Justice Anthony Kenneday and Chief Justice John Roberts. Justice Kennedy sometimes votes with the liberal justices, but more often, at least of late, with the conservatives. Chief Justice Roberts is conservative and usually sides with the three, but generally tries to apply the constitution, and if he can't swallow the illogic of Scalia, Alito and Thomas, sometimes breaks ranks. Judging from their past records, I would guess that at least one of them will vote to strike down the individual mandate, and I would give a 50/50 chance that both will. However, I think it very unlikely that both will strike down the entire A.C.A.
So I guess that concludes my prediction: I believe there is a 50/50 chance that the individual mandate will be upheld/struck down and a very small chance that the entire A.C.A. will be scrapped. But most likely, most of the A.C.A. will be upheld. I know, I hedged my bets with odds, I am both a statistician and a lawyer after all.