Orin Kerr at Volokh Conspiracy actually attended the Comstock argument and had these thoughts about how things went:
Just based on my recollections of the argument, I thought SG Kagan made a much broader Commerce Clause argument at oral argument than was made in the Government’s brief. Indeed, her argument struck me as sort of shockingly broad: She argued that the Constitution gives the federal government the general power “to run a responsible criminal justice system,” and that anything Congress plausibly thought a part of running a “responsible criminal justice system” was within the scope of federal power. Justice Scalia would have none of it, as you might imagine, but I couldn’t tell if he had any other votes.
My guess from the argument is that the Court will uphold the statute on the narrower grounds offered by the Government’s brief. If I had to make a more specific guess on a vote and opinion assignments, I would guess that it ends up being 7–2, with Scalia and Thomas dissenting.
I completely agree with Orin's substantive take, but I took away that Sotomayor was more skeptical of the government's argument. Either way, I was shocked (as I have been throughout this case) with the government's over-reaching in terms of federal jurisdiction. At many different stages, the government could have found a compromise position that it could have won, but it has clung to a view of the Necessary and Proper Clause that amounts to a general police power.