Well, to begin with, I don't know whether there is "good" CEC.... CEC is what it is, and that is because it is based mostly on soil type.  However, to answer your question, I would consider a CEC value between 10-15, maybe between 7-20, to be "good".  Why?  Because soils with a CEC in those ranges have enough capacity to hold nutrients. 

Just so we are clear, I would not consider soils with CEC outside those ranges to be "bad".