It's a must in the sense that it is truly a Disney classic. It originated at the 1964 World's Fair and is one of the few attractions in the Magic Kingdom not based on a Disney film. It has a wonderful sense of artistic style, but a very catchy song that can get stuck in your head for days on end. It's also really nice to see a grand Audio-Animatronic ride, something that Disney doesn't go for these days.
When I was a kid, I marveled at this. As an adult I've found the most joy in it riding in a boat with small children. If I'm in a boat without kids, about half way through I find myself thinking "GET ME OUT OF HERE!!!" But it's riding this with children, even if they aren't yours, when this attraction really comes alive to. I've never been on a ride where kids get such big eyes and a sense of wonder as they do on "it's a small world."
That, and the thematic element that even through the world's diverse cultures we are all one people is what makes this a great timeless attraction and worthy of its classic status in the Disney pantheon.