I've played KH and a bit of the FF series.
The KH games are great! The storyline for the first (PS2) is particularly appealing, and the ending leaves you wanting more.
KH: Chain of Memories (Gameboy) is definitely interesting, as it brings in more characters which make up the 'enemy' in KH2 - Organisation XIII.
KH2 (PS2) is a massive game, with a great storyline. More worlds have been added, more characters have been added, and the battle system has changed quite a bit. Personally I think the storyline for KH1 is better than that of KH2, but I did blast through 90% of KH2 in a weekend, so I've more than likely not taken it in as much as KH1 (six months!). When I play it again I'll probably take a lot longer, and take a lot more in.
Also, KH3 has been anounced. Tetsuya Nomura (FFVII director, FF vs XIII director, and the man behind everything Kingdom Hearts) is of course going to be directing it, with the same team who made KH1, KH:CoM, KH2, and Advent Children (not to mention Dirge of Cerberus), but work on it won't start until FF vs XIII is completely finished.