If you liked it for PSO then you'll love PSU. I've been playing the story mode on 360 because i wanted to get a full feel for the game and my internet was down all weekend. :x
Character customization is freakin crazy, you can change the height, weight, skin color, hair color, everything. You can even choose the pitch/voice of your character when they attack. They even have npc's who change your hairstyle and skin tone if you ever get sick of the way you look haha.
The storymode is like any other dungeon/RPG game, kinda corny but good enough to keep your attention. Personally i think PSU plays better than the new dot hack. After playing story mode i really want to get into the online one.
I havent had that much experience with the online portion of the game but i know that the 360 has different servers from ps2/pc. And you can also move freely from server to server if anytime you want(!), which is d-y-n-a-m-i-t-e.
