I've read up to volume 16 of Shaman King. In this volume, the Shaman Fight continues and Hao royally pounds one of the shaman teams into bits. There's also a revelation about the exact nature of Yoh's relationship with Hao - we've already found out that they're related, but now we discover just how closely related they really are. Oh, and I have to say, Hiroyuki Takei has included the ugliest baby imaginable in this volume. It's like a little goblin or something. Also, I think Lady Jeanne, the flaggelant who goes overboard by living inside an Iron Maiden, has a bit of a crush on Yoh, which is weird and a little creepy.
With most manga, I would suggest starting from the first volume, if possible, or from as early a volume as you can find if the first isn't possible. Starting at volume 16 with Shaman King would be a bad idea, and if you end up not liking this manga (or any other that you don't start with an early volume), it's not necessarily because the manga is bad.
In some ways, I kind of liked the early Shaman King volumes the best, before the Shaman Fight started and things began to look like Yu Yu Hakusho (Yoshihiro Togashi) or something. In fact, if you do like the Shaman Fight volumes, I would suggest trying Yu Yu Hakusho. I liked the idea, in the early volumes, of a young shaman-in-training who attends school like everyone else, even though he likes to hang out in cemeteries and can see ghosts. As the series was then, it looked like Bleach (Tite Kubo) for the younger set, only with a much more laid back hero. However, I am interested in seeing how things go with Hao, and whether there will be any revelations about Hao's motivations that will rock Yoh's and his friends' world.