Kingdom Hearts was one of the best games I have ever played; the first good action JRPG I had ever played. I really liked the platforming (hence the high jump and glide) and the extras, such as The Ice Titan, the Arena, Sephiroth, Phantom, Kurt Ziza, the 101 Dalmations... So much to do and explore after the game. AND, it wasnt linear. You could, and frequently did, get lost.
Kingdom Hearts 2 fixed the combat and camera, making them more presentable and fixing the gameplay. The plot in 2 excelled; but made the series feel like the Disney part was un-needed. They also took out a lot of extras; Sephiroth was the only extra, and the Arena was lame. Drives, Limits, etc. were fun, but I missed the extras and it was too linear.
Chain Of Memories was like a GBA remake of 1, replacing "Heart" in text with "Memory", except the parts when you were actually in Castle Oblivion. The combat was very fun, for a card-based game. Gave lots of variety, and the plot was interesting. Riku got his story in too, something I liked. I heard the remake of it is better; I have yet to buy it. Already own the GBA version.
358/2 Days was very interesting as well. The combat mimiced a mix of the three games: You had to build a deck with abilities, Keyblade and Magic (instead of having AP). They took away Mana, so you had a limited number of casts, based off your deck. Other then that, the combat was like the Original. If your health got to a point, you could activate a special limit, allowing your character to use a desperation move. The levels were linear, but they were intended to be, since it went for 358 days, day by day.
Playing as Roxas, seeing his story, and Xion made the game worth it. I loved the end of that game, and liked how it transitions into KH2 so smoothly. The multiplayer and mission mode were gimmicks, but fun none the less. Playing as the Organization, Xion, Riku, Sora, Donald, Goofy, or the King is immensely fun. The game was kinda slow, but forgiveable.
Birth By Sleep appears to be fun, with interesting combat and plot. I dont know much Japanese, so Ill wait for a localized version to come out.