PSYCHO is great for any time. The sequels are irrelevant. Definitely one of the greatest Horror films.
They may be irrelevant if you wish, but Psycho II is just as good.
I can see liking PSYCHO 2, but "just as good" really surprises me... to each his own, I guess.
I enjoy it as it takes a film where the great threat is a nail-biting, hidden physical presence and successfully evolves to a film where the threat is (Seemingly) clear and psychological. It takes our established fears and turns them to sympathies all the while maintaining something of the classic frightening atmosphere; there is less suspense, but what is taken is traded for tension and mystery.
In my mind Bates was by far the most engrossing character in the original, with others being little more than confused victims. Psycho II adds dimension by deepening character development as we see a sane but fragile Norman, attempting to reason with past madness as well as cope with a society who sees him as a
thing.
I feel Psycho and it's sequel are two very different films, but both are incredibly effective in their own ways.