The graphics are a problem; the gameplay isn't...
Disgaea 3 is exactly what most people are claiming it to be; it is the best installment of the Disgaea series in terms of features and the new aspects really help to stir up the combat more than some people are making out.
The puzzle element that has been enhanced dramatically since the prequel means that you are now essentially playing two games rather than one in a lot of cases. On the one hand you are playing a crazy and insanely fun turn based rpg, and on the other you are playing something that resembles Boom Blox. The puzzle element really forces you to think beyond the typical rock, paper, scissors aspect of what these types of games typically boast.
I like it a lot but if people are honest some higher rez sprites would have been nicer. The graphics don't spoil the game by any means but I hope for number 4 they actually do rework all the character models as if they don't update by then; it will look very dated in deed.
Bottom line; if you like this series or this genre then go play this game. Please don't let the graphics put you off as it is even better now than it ever was.