Katamari Forever is probably best-suited to players who are unfamiliar with the franchise, or, at least, players like myself, who loved the original game but didn't stick with the series. If you're a die-hard Katamari buff, you'll probably recognize more of the reused levels, which will make the game a little less exciting. But the concept remains the same. You use the two analog sticks via a tank-like control scheme to roll a ball around an environment. Items smaller than the ball stick to it, causing the ball to increase in size, which in turn makes it able to pick up larger objects. In its basic form, you're just trying to roll a katamari of a specific size or larger before the time limit expires. But several of the levels play around with the format a bit, such as giving you a burning katamari and forcing you to roll up fuel to keep the fire going as you try to get big enough to torch a specific target. Or another level that uses the katamari as a sponge. You'll soak up water in pools, then use that water to bring life back to a desert. The diversions from the main task of rolling up as much as possible are kind of clever, but the main charm in Katamari Damacy's gameplay comes from seeing some huge thing, and working yourself up until you're big enough to take it with you.
Backing up all of these rolling activities is another silly story involving the King of All Cosmos. This time, the King has fallen into some sort of coma. To ensure that the kingly duties are carried out, the Prince (that's you) builds RoboKing, a moping robot that instantly goes haywire and destroys all of the stars in the sky, then demands that you help him put them together by rolling up objects into katamaris. You'll switch masters between levels as you'll also be assisting the still-slumbering King to regain his memory. You'll also get some non-sequitur cutscenes, just for kicks. The (all-text) dialogue is also charming and goofy, and there's also plenty of really great songs--most of which are various remixes of the game's theme--to listen to as you play.
Katamari Forever also has online leaderboards and a local two-player option, but the game's at its best when you're just sort of rolling around, semi-aimlessly, grabbing anything and everything in your path. The levels are saddled with time limits that prevent you from grabbing it all, but you can eventually unlock a limit-free mode, as well as another mode that makes your katamari roll super quickly. It's not a huge departure for the series or anything like that, but if you're open to the idea of a new game in the Katamari franchise, you'll find that Katamari Forever fills the void quite well.