This is a prequel which actually works and explains things rather than giving even more questions. You can argue weather the mystery of Sadako is better left untouched, but the fact still remains; this is a good movie.
The lead (Yukie Nakama as a shy Sadako) is very good, even though it's obvious she's a Japanese version of Carrie White; misunderstood, creepy and with gifts she never wanted. That's the only real problem with this movie; the original ring movie was just that; original. This movie however, get's inspiration from other sources, such as Carrie, and that removes some of it's power.
While it's not as moody as Ring, Ring 0 still has some very good horror scenes and some smart camera-work. Seeing a white shape just at the edge of the screen is much more effective than, for example, the killer's POV.
As with Ring, this is a bit slow but builds up to a good finale. There are no special effects really, and it's very down-to-earth, one of the qualities of this and Ring.
Finally, there's the fact that it's Japanese. There's something refreshing about seeing a movie in a different language, in different environments, in a different culture.
I recommend this movie to those who liked Ring. It's definitely a worthy prequel. 7/10