Completely unique, a world of its own, incredibly imaginative
If you're curious as to what is so weird and interesting with this game, a lot of it has to do with the general weirdness of Japanese culture, as this game both visually and conceptually seems like it was taken out of a Japanese kids cartoon. The music is left as is, and all the songs are sung in Japanese, which really does a lot to add to the intrigue and otherworldliness that is consistently present. The dialogue also feels like it was almost purposefully translated as literally as possible, to add to the goofy atmosphere, which in effect has made the King of All Cosmos one of my favorite video game characters ever. And finally, the idea behind the gameplay, which consists of rolling up random objects (emphasis on random) in proportion to your katamari ('ball' or 'clump' in Japanese) and building it up in size as quickly as possible, seems like something that was yanked out of a warped child's imagination.
There are plenty of reviews here that explain the premise and the mechanics of the gameplay more in depth, but the main thing I want to convey is the sheer amount of fun and bizarre atmosphere this game creates. It is completely unique and a world of its own, and is really worth picking up.