I remember a guitar teacher(berklee graduate...whooaaahh)I had that was knowlegeble in theory and had decent technique.She asked me what I wanted to learn and I quikly replied 'far beyond the sun' or 'black star' from yngwie and I was totally capable to learn either.She said he was too fast for her and in a dignified manner made me realize she didnt have that skill to play let alone teach such complex(fast)music...needless to say I soon left.
Teachers are good if you want to learn theory or get some direction in styles you dont play or like,but thats about it.