The story can be forgetful at times; but the game play is where it shines the most. You can spend upwards of 20 hours just playing the main story. New game plus, and unlocking all the sub classes can take an additional 30 hours, and trying to attain all trophies in the game may take around a total of 100 hours or more. I really didn't have trouble with seeing the 'game over' screen, since I did grind my characters' a bit every other episode. It does help quite a bit if your characters are a level or two above the current opponents you're facing (level scaling is not present here thankfully), so you can grind to your hearts content.
Regarding extras, there are a plethora of side quests that you can do which offer a melange of new characters for you to recruit. Best of all, your characters' and their levels do carry over in NG+, including the equipment they current have; inventory and money as well.
Overall, the Disgaea 3 is an excellent JSRPG (Japanese Strategy RPG), and well worth the dollar to hour ratio you invest into.