In general, I endorse the is/ought distinction, in law as in other places. When Orin tells you what Supreme Court doctrine says, it doesn't mean he endorses it as the correct doctrine, the correct understanding of the Constitution, or whatever.
However, I think the line may be a wee bit fuzzier than Orin lets on. As an initial matter, consider that, under many constitutional theories — which many people subscribe to — the Constitution "means" something independently of whether courts have recognized it. People holding such theories can differ on what that meaning is, and on what interpretive theory you should use to figure out what that is — for instance original public meaning or something else — but these people would each agree that there exists some meaning of the Constitution. (Incidentally, I'm taking no position here on whether that view of a True Meaning is correct. This is an "is" point, not an "ought" point! In any event, I could phrase my argument without recourse to such views; but it's simpler this way.)
So, when we ask whether the Commerce Clause allows regulation of intrastate marijuana, one can give an "is" description of what the Commerce Clause means under one's preferred theory (e.g., "no it doesn't because the original meaning of 'interstate commerce' didn't cover that"), but that description would at the same time often be an "ought" description because usually such arguments would be accompanied with an argument that the theory is the normatively correct one. So such a description would be both "is" and "ought" at the same time, and more importantly, it could differ radically from what the Supreme Court says the Constitution means.
O.K., Orin might say — if I might put words into his mouth — but, he might continue, when I'm describing doctrine, it's clear what I'm doing — look, I'm citing Supreme Court cases! So (says the hypothetical Orin) let me translate my point into your rhetoric: Don't confuse "an argument about what current Supreme Court doctrine is" with "an argument about what the Constitution really means in your view, which really comes down to what you think Supreme Court doctrine ought to be."
Fair enough. But it may be misleading to claim that "Supreme Court doctrine" is the same thing as "constitutional law." For instance, it's also a view held by many that everyone who takes the constitutional oath — say all public officials — also have an independent duty to follow the Constitution. And there's no necessary reason why the Supreme Court's pronouncements on what the Constitution means should be authoritative. They're just one interpreter (well, nine) among many possible ones.
Now you'll probably want to avoid doing what the Supreme Court says is unconstitutional, in a context where they're just going to reverse you, just like you want to avoid walking down a dark street in a dangerous part of town — it's probably prudent, but it doesn't mean you accept it as the way things should be. (But not even everyone agrees with that — some argue that there's a duty to follow your own view of the Constitution, even if it means reversal, because that's just your duty as a defender of the Constitution.)
But there's no reason why you can't take a stricter view of the Constitution than the Court. For instance, as a legislator, you might say: "The Supreme Court says this law is constitutional, but I disagree, so I'll vote against it on constitutional grounds." Or the President could use the same argument for vetoing a law, or for enforcing a law in a particular way. (Some have also written about the President's duty to veto, and the President's duty not to enforce unconstitutional laws.) Or regular folks could use their own view of the Constitution in making arguments to these officials.
This stuff rarely gets litigated. For example, regular people don't need to account to anyone for the arguments they make; elected officials don't need to give reasons for why they vote against or veto something; and so on. But these are still arguments about the meaning of the Constitution, or, if you want to put it differently, about the substance of constitutional commands.
Now, is this constitutional law? Because, some might say, arguing about сonstitution meaning isn't the same as arguing about constitutional law. For instance, in the view of some, "law" is just what comes out of the courts. (Perhaps constitutional law, in this view, should be influenced by constitutional meaning, but it might diverge from constitutional meaning sometimes, either for good or for bad reasons; but the two are different things.) I disagree, and so does a substantial literature on "popular constitutionalism." I won't try to give a complete definition of "law" here, but I'll say that at the very least, legal principles include any principles that purport to bind government actors in a way that purports to be independent of mere policy preferences.
For instance, this is a legal argument: "You, Senator X, have the duty to vote against this law regardless of how good an idea you think it would be as a matter of policy, because it's contrary to the true meaning of the Constitution, by which I mean the original public meaning."
And if Senator X himself claims, "I think this is an excellent idea, but sadly I feel bound to vote against it because it's contrary to the Constitution, even though the Supreme Court would uphold it," he's enunciating a principle of constitutional law, though not a principle of a judicial variety, and not one that could actually be enforced in any court.
An implication of all this is that if Senator X votes against a law because he (correctly, in your view) thinks it's unconstitutional, and the law gets enacted anyway, and then the Supreme Court (incorrectly, in your view) upholds it, then it's perfectly valid to say that Senator X understands constitutional law better than the Supreme Court does.
What this all comes down to is that when Orin makes an argument about the state of current Supreme Court doctrine, you should listen to him and not confuse it with an argument about what Supreme Court doctrine ought to be. But you also shouldn't think that Supreme Court doctrine exhausts the whole of constitutional law. And in particular, if your argument about what Supreme Court doctrine should be is grounded in some theory of constitutional interpretation, your "ought" argument can also be an "is" argument about constitutional law, though not about the Supreme-Court-doctrine branch of it.