I have seen it many times- if someone doesn't get it they immediately scream that it isn't art. I have heard that Klimt isn't art, and neither is Picasso or Wassily Kandinsky. It's always immediately followed, in my experience, with 'I like Real art. Art that looks like something. This modern art is junk.'
Sometimes, when I hear that I am tempted to tell the person that their viewpoint refreshes and enriches me and that I will immediately become devoted to John Currin's work and buy as much Thomas Kinkade as my wallet will bear. It is a life-changing experience, to learn that someone who once spent ten seconds looking at Yves Tanguy's Indefinite Divisibility can dismiss every piece of art produced since the year 1900 as trash.
It is true that there are artists whose art I don't enjoy, that I think is devoid of any substansive meaning, that is trite commercial crap. But that is okay. I don't have to buy it, or look at it. There is an entire world of artists busily creating art and all I have to do is look. The sheer number of artists creating means that there is some art being made right now that will cater to any taste.
Telling someone that one hates all modern art is exactly like saying one hates modern music. Which modern music? Britney Spears? John Cage? autechre? The Pillows?
Also, the reason that many people have the misconception that art was 'better' in the old days is because in the centuries since art critics and curators have had time to decide what was good art and what was forgettable. When going to a museum and looking at older pieces one is looking at centuries of debate about what a piece is worth, artistically. And the symbolism of much of that work is lost. When looking at those billowy women draped in gauze wearing improbable helmets remember that every piece of that work is rich with symbolism.
Just like Guernica.
Just because you don't understand it doesn't mean it's not art.