Shaman King Zero is a story based upon the past and on the "be-comings" as to how each character was created (his/her persona). Shaman King Zero was released after the series itself ended, with a craving for more, this was also amplified due to the fact that it was cancelled and had to be wrapped up immediately.
The story as to each character gives us a stronger connection to each of them, if anything reading them after is just to connect us and empathise with each of the characters more, especially Yoh-Asakura. If it was recommended to read these before hand (by a friend
or someone else that has read this series), since they provide the stories to everyone's character BEFORE the shaman fight had even begun, forget it. DO NOT DO SO! That is my recommendation. It will wreck the series for you as you will also be introduced to people you have no idea, and will not face until later on during the manga series.
It is quite an enjoyable six chapters. As it includes an extra chapter which is:
Shaman King Zero: Chapter 5.5
Overall, this manga series has again proven Hiroyuki Takei's talent for storytelling and his art. It may possibly however just annoy you even further with the series since it's not giving any comfort or closure to the end of the Shaman King Original Series. It is a great use of time if you were extremely addicted to the original series as I was. Have fun, and give it a go if you enjoyed the original manga series!