Overall
I might have got the chronology wrong, but I started out with this OVA, and have yet to see "Sign" and "Twilight". That's probably why I didn't understand a thing.
Animation
Not bad, not great. It looks kind of like most TV animes out there. I guess you expect more from an OVA, though. I like the attention to detail in the animation, though (throw a frying pan down a hatch, and it's still there when you get down there).
Sound
Voice acting is solid but nothing special. Background music is a bit repetative and there are few different songs considering this being a rather short OVA.
The ending theme gave me a real deja vu. I can't put my finger on it, but I have heard it before somewhere. Are there other animes using this song?
Story
Alright, I probably should have started with "Sign" or something... I didn't understand a thing! Yes, there are strange things going on regarding "The world", but what? And why is this rag-tag collection of characters seeking the truth?
Hopefully this OVA will make more sense when you have watched other parts of the .Hack story.
Character
As I said, I can't see how the characters' backgrounds link together, and certainly not how they are connected to the story (which I also failed to understand, by the way). Mai-chan is the only one with a real motive as far as I can see. Even though every character gets a whole episode of their own, there are no explanations of who they actually are and why they do what they do.