I got my copy last Thursday and I've spent my whole weekend playing this game; so much I could do in the game and I'm neglecting the main story. I'm sill in chapter 3 after like 3 days playing, spent too much time in leveling characters and items, and of course CoO. Unlocked THE grind mode CoO 4 while I'm still in chapter 2 thanks to the introduction of Cheat Shop(First introduced in Disgaea D2 IIRC, not in original D4 on PS3).
I'd say this game is really made for Vita, enjoying playing this game more than on PS3. Overall game is great but there are some stutter mainly in each 10 level of item world. Game slowed down quite a lot there as the map is quite huge, I don't think I experienced the same with Vita D3 tho. Other minor problem I accoutered in item world is the Mystery Gate loading time. This is due to some room require Internet connection(I believe it could be disabled). I was at continuous IW run, run into Mystery Gate and it then it goes blank, I waited for 15 seconds without any apparent activity and I thought the game hang, however after a few more seconds, the room loaded. Apparently the game connect to Internet and downloaded other players data. Game should have showed some loading message instead of pure black screen.