Here's the deal:
Story: Interesting, but repetitive. The best "Story" part is the final 3 missions (Groudon, Kyogre, Rayquaza) but even those are too easy as there's a pattern to all of it. Creative characters and childish jokes, but kinda dissapointing.
Gameplay: New, creative, but lame and infuriating. I'd rather beat a Pokemon in battle than circle it seventeen times and learn it's totally useless to me. Bonus features: Freakin' sweet. Extra story which is better than the main stuff, and of course, the Manaphy egg.
Reason to get it: Manaphy and Fione. Because unless you're a hardcore Pokemon fan like me, this game is not that fun, and you gotta be biased to enjoy it.