While obviously not a 'proper' Kirby release, it stands on its own as a great new addition to the series.
Speaking of the various worlds, the level design is quite creative. Stages are quite dynamic, with examples such as a music level where you jump on certain instruments, or swing across them, or the myriad of minigame-like stages, where you can race as a buggy, dolphin or surfboarder, or even turn the game into a SHMUP temporarily! Plenty of variety here, and the quality of each stage is good, and spaced out well to prevent any monotony.
For some replay value, there are quite a bit of collectibles. You can play 'Kirby Home Decorator' in a way (Funner than it sounds!) getting various furniture or fabric items to decorate your pad. As well, you can decorate rooms for extra tennants in the apartment, who you can play some simple minigames with. Finally, there's CD's you can use to listen to the in-game music.
The music itself is quite a treat. Most of the tracks are new compositions, with a pianos and soft percussion making the entire soundtrack fit the game like a glove. Once one gets to the Dreamland stage, you'll find quite a bit of classic tracks remixed, notably including Yin-Yarns theme, which primarily remixes the Kirby 64 boss theme, but also uses plenty of previous villains themes as well.
There is one niggling little complaint that longtime Kirby fans probably will have however, and it's somewhat warranted. This obviously is not a Kirby game. Originally it was developed under a different title without Kirby, and the core gameplay is radically different than what you'd expect out of a Kirby game. There's enough there for nostalgia purposes, but this isn't the home console release Kirby fans have been expecting.
And lastly, a note for newcomers, this game is quite easy. You don't ever die, just if you get hit, you lose gems and don't get as good a rating for the level. Completing the game 100% takes some skill, but it isn't overly taxing. Nothing new for Kirby fans however.
In summary, an excellent little ball of good design, play it if you want to relax and feel happy. =D