Is Birth by Sleep a good game on it's own or does it purely appeal to people who are into the KH franchise?
How is the battle system compared to original KH?
Back when KH came out, the camera controls and lock on implementation in that game was such an annoyance to me that I finished it as fast as possible and traded it in so I'd get max credit for it. The camera/lock on problems irritated me so much that I wrote off the series and skipped KH2. All these portable games seemed so confusing and such and since I wasn't interested in the story, I wrote them off too.
I suppose if you say the battle system is absolutely amazing (and there are no camera/lock on problems), then if they ever put it up on PSN, I'll have to try it out.
I'm late responding to this, but I feel very strongly about this game. It's really, really good. It's basically the only KH game I completely adore--the rest all have annoyances that bother me in some way or another (usually minigames or stupid story or horrible mechanics or game structure). And on Critical Mode, BbS is one of the best action RPGs out there. I don't want to write a lot, so I'll try to keep this as short but informative as possible.
lunch's negative impressions are spot-on: the game immediately opens with some cringe-worthy melodrama about friendship, but it actually ends up having one of the better stories to come out of the franchise. Because it's a prequel that basically feels like KH0, a lot of it is self-contained. If you played Odin Sphere or something similar, the crisscrossing stories are kind of like that. I was surprised to find myself actually enjoying how the game touched upon some uncharacteristically dark themes. The worlds you visit are also symbolically related to what each character is going through.
But you really don't play it for story. BbS's battle mechanics are the best KH has ever had, and all the auxiliary progression systems are addictive and thoughtful. In my eyes, the PSP had Birth by Sleep, and the DS had Order of Ecclesia. Both games have some outstanding boss fights. Regular encounters also generally require more strategy in BbS on Critical than other KH games, especially near the beginning. I don't know how everything holds up on the easier difficulties, but BbS is very easy to recommend if you enjoy a strong, cohesively-designed battle system that caters to your personal playstyle preferences.
Comparing BbS's battle system to KH1's is really difficult because they're totally different. For one, you don't have Goofy and Donald with you; you're usually fighting alone. You have an arsenal of abilities that you can equip (Command Deck), and everything works on recharge times instead of MP. Using certain commands will put you into a limit break mode of sorts (Command Style) that can eventually take you into another limit after that, provided you fill the meter for it by continuously attacking. You also have new things called shotlocks which are first-person lock-on attacks that render you invincible for a brief period of time. There are also D-Links that replace summons, briefly changing your Command Deck to one inspired by another character while also fully healing you. It's really bizarre to explain, but just trust that the battle system is very good. On Critical, you will be taking advantage of everything at your disposal and abusing stuff like frame traps and animation invulnerability. The camera is also less annoying than it usually is in KH games (I found KH1's camera to be one of the worst, actually), and dealing with lock-on and camera rotation will eventually come naturally after you get used to them. It's not perfect, no, but you won't have OG Xbox Ninja Gaiden camera flashbacks or anything. At least, I didn't.
It was probably a good thing that you wrote off the portable games, too. I'd consider KH1, KH2, CoM/re:CoM, and Dream Drop Distance to be more worth your time than 358/2 Days and Re:coded, but BbS easily bests them all in my humble opinion. It's a shame that isn't (and won't likely ever be) on PSN because people are sorely missing out on one of the PSP's best titles.