Kingdom Hearts is a big series for Square Enix. With over 14 million copies sold worldwide, the series is the rare action RPG that's as popular in the US as it is in Japan. Kingdom Hearts, Kingdom Hearts 2, and 358/2 Days have each surpassed 1 million copies worldwide, and Birth By Sleep has sold over 830,000 units across its two versions.
With a 3DS release on the way and ever-present rumors of Kingdom Hearts 3, the franchise still has support and a die-hard fanbase to back it up. Kingdom Hearts doesn't have the sales numbers of Final Fantasty or Dragon Quest, but thanks to its crossover nature and real-time combat, the series is successful enough to have a place on store shelves.
From IGN.com
Japan's Biggest Gaming Franchises - Games Feature at IGN
Not bad for DDD