I have almost spent 100 hours to get 100 % on all 3 characters. Do not compare it with the DS games. While they felt more like a side story BBS feels more like a full game. It plays like the PS2 games and is one of the best PSP games.
To be honest, Square-Enix has some brilliant games on the PSP. Crisis Core was one of the best games in that year and overshadowed most PS3 games from that year storywise... the ending scene was one of the strongest I have seen so far and playing the Nibelheim incident in modern graphics was thrilling....
Final Fantasy Dissida also was a nice game and showed the power of the PSP. I must admit, that I waited very long before I decided to buy a PSP. I already owned a DS for some Square-Enix games and my wife couldn't understand why I would need a second handhelt. But the PSP just blew me away.
Since I am mostly a "story-gamer" i never got much satisfaction out of handhelds. The GBA was nice because it got many SNES remakes and Square-Enix put some remakes out. It was a good chance to pick up FF4-6^^^. Then the DS came and the touch screen made it unique... but in my opinion it was more a nuisance than a help. It is hard to play with the stylus and the buttons at the same time, so I mostly go buttons only or stylus only... and only few games could make use of the stylus while adding something to the game... (for example FF 12 Revenent wings or The World ends with you).
The PSP was the first handheld I bought that brought games that felt like real console games. Especially the Japanese Companies like SquareEnix, Capcom, Konami and Sega really put much effort in their PSP games. Add in some first party games like Ratchet & Clank, Resistance or God of War and some awesome PS1 Ports like Valkyrie Profile, Symphony of the Night or Final Fantasy Tactics... I must admit that I really like my PSP and it is now really hard to go back to a DS game (though there are some unique DS games like Professor Layton...).
The Reason may be that I have the feeling that developers simply put much more effort in PSP games than in DS games. Just compare KH Days with KH BBS, FF 12 Revenant Wings with FF 7 Crisis Core... Konami is even putting Metal Gear on the PSP... and I am sure that FF 4 complete will be far better than FF 4 on the DS.
So I am pretty sure you will enjoy KH BBS very much. Just don't use the DS games as quality measure... BBS is far ahead and can stand up against the PS2 Games. Too bad the next KH will come to the 3DS instead of the PSP2... I expect that the quality difference between 3DS and PSP2 will stay the same and the PSP2 will so more commitment than the 3DS over the years, even if the 3DS will sell more Software and Hardware overall.