Actually, all of his post-Strangelove films got a lot of mixed to bad reviews upon release. 2001 was panned both by The New York Times and Pauline Kael. Their stature has risen with the years. 2001 is only boring if you need constant narrative stimulation. Its’ form fit and enhanced its content.
I guess I must have missed Kael’s comment. I never read the Times anyway. Most of the reviews I read (trying to stir faulty memory) praised the flick. But I’ll demur...even with constant narrative stimulation, I think I’d find it pretty tedious.