The soundtrack for .hack//sign is one of Yuki Kajiura's early works, and it is absolutely amazing. When it came out, it was unlike almost anything else. (Noir is another early work by YK, and also very good). You should absolutely buy the soundtrack. Don't bother watching the anime.
The music is especially noticeable, because there is almost nothing else that will grab your attention on screen. The background art is decent, but most of the animation simply reuses the same sets over, and over, and over again. The first few episodes make you think that the anime is going to turn into something really cool, but it never pans out. The plot is entirely uninteresting, and the characters are either really annoying, or really boring, and often both.
The anime is supposedly set in an online MMORPG (kind of like Sword Art Online), but it has got to be the most boring game ever made, because we almost never see any of them actually play the game. Instead, they mostly sit around waiting for things to happen. Of course, nothing ever does.
All that would be okay if we had great characters and dialog, but the characters are entirely meh. The authors attempted to make the dialogue deep and mysterious, but nothing of importance is ever said or revealed, because nothing of importance ever happens in the plot, and the characters don't develop in any meaningful way. There's a little bit of action at the end, but after 20-something episodes of just waiting, I had long since ceased to care.
