Sure, The Amazing Mirror had one too many multiplayer-only features in certain sections of the game, but it's a Metroid-vania Kirby game. That's all anyone needs to know. It's got tough bosses, a giant map and plenty of cool co-op sections, if anyone can actually find someone with a Game Boy Advance and a link cable. The co-op sections aren't necessary to beat the game, but it is a cool gimmick that unlocks new parts of the overworld. There's also a lot of geeky mini-games for parties or something.

Kirby frequently tries something different. In fact it may just be the most experimental series to date. There is a puzzle game, a racing game, the interesting mechanics of Canvas Curse and it's spiritual successor, Mass Attack. There is even a (really fun) golf game.
It's the alterations between Kirby's main platformer series that gets attention, and Amazing Mirror fits this bill. Going for a Metroidvania feel, Kirby and the Amazing Mirror is a blast to play for the same reasons as Metroid and the newer Castlevania's. However, by throwing in Kirby's abilities and puzzles relating to said abilities, you have a game that, I think, outperforms them in every way.

Now if it only was on something else other than GBA...

This game is cool, but so unpopular. It's like Metroid and Castelvania in a Kirby game.