It's never come before SCOTUS directly (a technical question about whether lawyers can speculatively sue for information about specific cases was heard in October and will be decided next year) but has survived in lower courts.
I'm obviously not an expert but the odds seem very high that warrantless electronic surveillance of specific foreign targets will be found constitutional; warrantless searches of Americans are constitutional in many circumstances.
It's important to remember that "constitutional" is not a synonym for "correct". There's a lot of things we could do that are constitutional but ridiculous.
Late edit:
Note that while it's hard to find FISA case law, it's not as hard to find case law on the principles animating it. Here's SCOTUS, US v US District Court 1972:
Moreover, we do not hold that the same type of standards and
procedures prescribed by Title III are necessarily applicable to this
case. We recognize that domestic security surveillance may involve
different policy and practical considerations from the surveillance of
"ordinary crime." The gathering of security intelligence is often long
range and involves the interrelation of various sources and types of
information. The exact targets of such surveillance may be more
difficult to identify than in surveillance operations against many
types of crime specified in Title III. Often, too, the emphasis of
domestic intelligence gathering is on the prevention of unlawful
activity or the enhancement of the Government's preparedness for some
possible future crisis or emergency. Thus, the focus of domestic
surveillance may be less precise than that directed against more
conventional types of crime.
No, because it has been very hard to even get a lawsuit against it accepted, either because NSA or the White House tried to dismiss the cases because they don't have standing, because they can't prove NSA is spying on them, or by using the state secrets privilege.
