Just beat the game. This game is amazing! There's so much creativity on display that it just beggars belief and this is reflected in the incredible amount of content squeezed into the game (12 hours it took me to finish! That makes it the longest game in the series by a country mile. I could probably finish every other game in the series back to back in the time it took me to finish Mass Attack! - Note, I have not yet played KRTD, so I don't know how long the levels are in that game)
But not all is perfect with in regards to length, because I think that it's length is also a bit of a curse. Many of the levels go on too long (about 60% of the levels are at least twice as long as a typical Kirby level!) I would've preferred to see them cut down and repurposed into more levels that were shorter in length (like DeDeDe Resort, which is by far my favourite world in this game!)
Also world 2 (Sandy Canyon) is a bit of a drag in terms of pacing. Not as many new mechanics introduced as the other worlds and too many overly long levels. The final boss is also a bit of a let down (especially in comparison to the crazy awesome final bosses that the series is known for)
But those are minor issues in the grand scheme of things. The controls are amazing, perfectly blending Pikmin style unit management with the High Energy Action that the Kirby series is known for. Never did I have any problems with getting each Kirby to do what I wanted them to do. The level design is superb (aside from the levels themselves being a bit too long), with a crazy amount of game mechanics constantly and effortlessly introduced in each new level (Mario Galaxy/DKC2 style!) So much creativity! The game always feel fresh and the mechanics are works of genius! (That Tree Level! YES!)
Also, the game puts up a good fight. It's by far the hardest game in the series and if you're going for 100%, this game will kick your arse (yes, a Kirby game that is actually arse kicking level hard, and I'm not talking about boss rush!) and you will see the game over screen more than once!
The soundtrack also deserves specific mention too. Almost no remixes at all!, save the death fanfare, miniboss music and one really unexpected (and totally awesome!) remix. Considering that the last Kirby game to feature a soundtrack that largely consisted of original music was Kirby 64 back in 2000 (Epic Yarn doesn't count since it wasn't originally a Kirby game), this is a VERY welcome change of pace! Speaking of which, the music is of a very different style to previous Kirby games (obviously because Jun Ishikawa wasn't involved. Shogo Sakai took the reigns instead) and it's really nice. Very memorable tunes for the most part, with some really odd styles mixed in!
Finally, the extra sub games. They really need to be mentioned separately because they're shockingly fully featured! Several of them could easily be sold on DSiWare as is! (I wouldn't be surprised to hear that Kirby Brawlball was a cancelled DSiWare game that was chucked in for the hell of it!) and they're all great fun!
An enormous accomplishment and a must have DS game, despite a few pacing issues. The handheld couldn't have a better sendoff!