Unique and crazy good. It make me want a PET for myself! :)
It's various networks (kinda like dungeons) provide both puzzles and fast-paced fighting action. The ability to collect, build, and then apply a variety of attacks allow you to switch multiple battle styles and gives you a reason to keep on playing, trying to collect more expensive and rare battle-chips and power-ups.
MMBN also has a very engrossing storyline which, in the age of technology, seems very probable and makes you want to have a PET in your everyday life. Plus, the gameplay is so natural that you can't put the game down because it feels so right.
Overall, MMBN is an amazing game for the GBA, possibly the best in the series, and is worth buying, even 8 years later.