I only do Item World to power up items, not to grind for levels. Although the two usually go hand-in-hand anyway. After a certain point I find that the items in shops aren't that great.
10 level 75s? Well if you have really good equipment on your lv50 characters I'd say it's possible at that level. Maybe. I'd believe it if there weren't ten of that guy, lol.
In Disgaea 2 there's this really good level with lv 300 monsters on EXP +50% geo panels. That plus tower attacks is awesome.
I gave up on item world, idk. It seems like such a pain to go through after a while, and I just stuck to normal grinding through certain storyline characters. Like, there's this one stage that I went through about 120 times, and for a good reason. The boss that I'm going to fight is level 75, and I'm level 50, so if I'm going to stand anywhere near a chance to beat him(and there's 10 duplicates of him, too), I'm gonna have to grind like there's no tomorrow. >___>
But the good news is, it does pay off with good money, which means better equipment, which means stronger characters. :D
I've only played Disgaea 2 myself, although I still want to get Disgaea DS sometime.
The sad this is, if you just want to beat the storyline, you don't have to grind. But I find that I want awesome weapons and armor, and I have a lot of characters to equip, I'm spending 100+ hours in Item World just to keep up. And for the postgame content you need the crazy-ass levels, too. :( I'm debating cutting back some characters, or just letting them sit and leveling a select few, so I can have a small army of like 4-5 characters that can beat anything instead of 10 that can't beat much. *shrugs*
I introduced my little brother to Disgaea 2 and he at least at one point played it for hours every day. (He also played Disgaea but the PS2 version.) It was at the point that he bought a PSP for the Disgaea 2 port of it. I don't know if he plays it that much anymore, though.