I have very mixed feelings on this issue. First, I have yet to see a shaman that can even come close to killing a well prepared warrior who knows what he's doing. In that sense I think they are very weak.
HOWEVER, after having the honor to group and see Atheakeetha and Tylor in action, I would be scared shitless if Shamans were any stronger. Put a knowledgeable Shaman with a warrior, and if they know how to play together I believe they are among if not THE most lethal two character combo on the mud.