Definitely a good game. Dynastic Hero is just a re-badged Wonderboy in Monsterworld for the Duo, so if you've played that game, you know what DH is all about. That said, if you already own Monsterworld there isn't a great deal of incentive to pick up DH given its price, except the excellent redbook soundtrack. Beyond that, all the basic gameplay elements are identical, with only a handful of cosmetic differences, such as the aforementioned music and lack of parallax in some areas. What DH lacks in parallax it more than makes up for with its much more vibrant colors.
I couldn't recommend anyone spend $500 on this (or any other game) and keep a straight face, but it certainly isn't a bad game.