Just like all the other people have said: it's fun, quirky, everyone who plays it falls head over heels... but there's a snag, and it's a big one!
The game is shipped with 6 of the 19 levels locked, and the only way to play them (and therefore complete the game 100% with all achievements) is to shell out 1400 MS Points (or £11.90 if you're using a credit card) to unlock the levels. What's more, buying these "extra" levels is not a download as they're already on the disc.
For this reason alone, I cannot recommend buying this game for anyone other than die-hard Katamari fans who have played the PS2 games to death already. As a matter of principle, boycott this version as it sets a precedent for "crippled" games on general sale. Imagine if Final Fantasy VII had required a £10 kicker before you could load the 3rd disc and complete the game, there'd have been outrage. What Namco has done here is just that, and their cynical handling of an extremely loyal fanbase needs to be given some real media attention.