Before I start, let me point out that the only exposure I've had to the.hack universe is through this series. So if I'm just not getting something that is somehow enhanced by the games or other series, I apologize.
Anyway, this is really a pretty good show. Not great, but good. Good characters, great animation and music, and an inventive and creative plot. The only real beefs I have with this show are the length and the confusion. During the series, I accepted the fact that things may seem a little confusing, and I'd just have to stick with it. I expected that things would be tied up at the end, maybe not completely, but at least enough to justify watching all these (mostly pointless) episodes. Unfortunately, I don't feel that they really explained what happened. It kind of left me hanging. I understand the story goes on, but I would have preferred a little more closure.
As for the length, the series contains far too little content to fill nearly thirty episodes. Its kind of an anti-FLCL, if you know what I mean. They could have easily wrapped it up in 13, if not fewer. It just seemed to drag on and on, talking a lot without really saying anything. Of course, sometimes it was nice to just sit back and enjoy the beautiful animation and music. They were something of a saving grace.
So in the end, I'd recommend .hack//sign. Despite its length, it managed to keep my interest throughout, and that's always a good thing.