It was a new concept and because people felt the graphics of the game were too "kiddy" or "subpar" it often gets overlooked. Many katamaricans feel that the graphics add that certain charm to the game. It's that aspect you don't see in games too often anymore. Many games today are so heavily reliant on being shiny and realistic. Katamari focuses on the game and not the presentation. That's not to say it's ugly but many fans say the game wouldn't feel like katamari anymore if it wasn't done in this graphical style.
The game and its gameplay lies in simplicity. You roll a ball, stuff sticks to it, it becomes a star, a planet, a meteor, an island, a comet, and you're done. You start out small, rolling up push pins and and matches and eventually work your way up to rolling up the same planets you're making for the King. Certain levels bypass the size requirement in favor of the "roll up a certain item" requirement. One such example would be the Flower Level in We Love Katamari where the goal of the level is to roll up a certain amount of flowers. Munchies Manor from Beautiful Katamari has you rolling up food and the goal of the level is have have as many calories rolled up as possible.
Many people get turned off by the concept of katamari because of what it is, but once you play it, it's hard to put down. The humor in the series, between the dialogue and the cleverly placed items, is something the fans love the most. It's that same over the top quality that the katamaricans have come to expect from the series and anything related to it. The King's ridiculous costumes, the new cousins, even level design keep players coming back to the series.
It has gained a somewhat of a cult classic status among gamers. There are those gamers who don't want to admit they like the series so much because it's, well katamari. But in the end, regardless of everything else, the game is successful and a fan favorite.