Well, since the game is split into 3 characters you'll eventually feel like "Ugh, doing this again..." but each character does have their stages set up specifically for them, like areas that only that character will see in each of the worlds. In that respect it actually is really well done.
KH2's level design was pitiful compared to KH1. While BBS isn't anywhere near as good as 1, you can see thought put into levels again (I don't mean backgrounds or graphics, I mean like platforming and exploration and hidden goodies). With that said, some of the worlds feel a little dry visually... but then you'll see another part of it on another character that ends up being really interesting.
The mini games in this are awesome! and plentiful, too.
The skill setup and development is really cool, but without a guide you'd be lost as far as skill fusion is concerned. If there was one thing that KH RE: CoM did really well it was make you feel super powerful. BBS does this without a /complete/ card system. Like, blasting down an entire army of enemies with a Mega Flare... great stuff. The limit breaks or whatever they're called (been a while) you go into also make your moves flashy and you'll feel super empowered. Shot locks are just evilly devastating to all but super bosses. What I'm saying is, the core gameplay is super fun and very flashy and it just works.
The story for the most part resembles KH2's mechanic: start with some story, have a bunch of nonsense in every world, and then shoot you down a vertical drop roller coaster ride at the final stage that blasts you with so much awesome that you really end up forgetting everything about the game except for those parts. The only exception to this I think is Terra's story, which is by far the best in content and the best pulled off throughout the campaign.
This game is amazing. Easily my favorite PSP game. Great story, awesome gameplay. Although I would not recommend playing it if you haven't played the other games in the series. The story will make no sense.
"The Arcana is the means by which all is revealed."